MacOS

Background
macOS is an operating system made by Apple, starting out as just "Mac OS" during the '80s. Originally being made as a development of the Apple Lisa interface, it debuted with the release of the Macintosh 128K in 1984. The operating system would later change over the time with various improvements. After 2001 with the release of version 10, the OS would change its name to "Mac OS X". With the release of Mountain Lion, the name was changed to "OS X". The name of the OS is currently "macOS", starting with Sierra in 2016.

1st Screen (January 24, 1984-February 1, 2002)
Screen: On a halftone gray background, we see an icon of a Macintosh with a happy face.

Variants:
 * Whenever the Macintosh has a severe error, (or happen to press the interrupt switch on the side of some Macs) It will display a black background with the same Image of the Macintosh, but it will be displaying a frown and has X's over its eyes. This is called the "Sad Mac", which was used in 1984 to 2002.
 * In Macs that run Mac OS 7 to 10.1 Puma, the background is gray.
 * Starting with Mac OS 8.5 to 10.1 Puma, the Happy Mac icon of the 1st logo appears, but colored. Starting with the release of Mac OS X 10.0, the Happy Mac became flatter and the spinning wait cursor (commonly known as the"spinning pin wheel of death") appears on the top right corner.

Technique: None.

Startup Sound (Music/Sounds): The trademark Apple note has been known to evolve during the course of the Happy Mac startup, these include: All these sounds can be heard here.
 * Macintosh 128K, 512K, Plus, SE and Classic (1984-1992): A beeping sound.
 * Macintosh II series, SE/30, Portable and PB100 (1987-1991): A synthesized guitar note.
 * Macintosh LC Series & Macintosh Classic II (1990-1996): A quiet synth note.
 * Macintosh Color Classic, Quadra/Centris Series and 68k Powerbooks (1991-1995): An airy keyboard note. Composed by Jim Reekes.
 * Power Macintosh LC & Performa 5200/5300 Series (1995-1997): A string chord.
 * NuBus Power Macintoshes (1994-1996): A live guitar note. Composed by Stanley Jordan.
 * Quadra/Centris AV Series (1993-1994): The trademark synth chime note that would later be used on newer computers. This was also composed by Jim Reekes.
 * PCI Power Macintoshes, PowerBooks (1996-1998): A lower pitch version of the same synth chime. PowerBooks 3400, 2400 and G3 Kanga have a shortened and slightly slower version of this chime.
 * 20th Anniversary Macintosh (1997-1998): An echoing light synth note.
 * PowerBook G3, iMac G3 & Power Macintosh G3 Blue & White (1997-2002): An even lower pitch and lower quality version of the Quadra/Centris AV chime. The same chime would be used on newer and current Macs.

Startup Sound Variants: When the Sad Mac is displayed, depending on what Mac you're using, a noise will be heard in the background. These chimes are nicknamed the "Chimes of Death": All of the chimes can be found here.
 * Macintosh 128K, 512K, Plus, SE, Classic, iMac, & Power Macintosh G3 Blue & White (1984-1992, 1998-2002): None.
 * Macintosh II series, SE/30, Portable, PB100 (1987-1993): A synthesized guitar arpeggio. Depending on the error, this succeeds either a short chord, a single note, or two notes.
 * Macintosh LC Series, Macintosh Classic II, 68k Macs, PowerBooks (1990-1996): A higher pitched version of the arpeggio played on a synth followed by four more notes. Depending on the error, only the four notes could be heard.
 * Quadra/Centris AV Series (1993-1994): A short, fast-tempo drum tune.
 * Power Macintosh LC, Performa 5200/5300 Series & Macintoshes with PowerPC upgrade card (1995-1997): A dramatic three note brass fanfare followed by drumbeats.
 * Power Macintosh 6100, 7100 and 8100 series (1994-1996): A car engine sound followed by a car crash.
 * PCI Power Macintoshes and PPC PowerBooks (1995-1998): An airy shattering/axe sound.

Availability: Appears on all Macintosh computers running System 0.97 to Mac OS 9.2.1 (when using a Old World ROM Mac). After the release of Mac OS System 3.0, this screen would appear before the next one, then before the 3rd logo in Mac OS 7.6.

2nd Screen (January 24, 1984-April 7, 1997)
Screen: On the same background as the 1st Screen, a bordered rectangle appears, with an icon of the computer and mouse on the top left corner and the text "Welcome to Macintosh." in the top center.

Variants:
 * An earlier variant exists where the rectangle has a drop shadow rather than a border.
 * In grayscale and colour monitor Macs that run Mac OS 7 up to 7.5.5, the background is gray.
 * In colour monitor Macs running Mac OS 7 up to 7.5.5, the computer icon is coloured.

Technique: None.

Startup Sound (Music/Sounds): None.

Availability: Rare. Appears on Macintosh computers beginning with the Macintosh 128k (System 0.97), presumably ending with the release of System 7.5.5 due to the introduction of the next startup screen with Mac OS 7.6.

3rd Screen (January 7, 1997-February 1, 2002)
Screen: On a gray background, a bordered gray square appears with another bordered square inside, filled in white, with an icon of a blue/cyan face with a large nose, which makes it look like two faces if you look closely, one of them looking at the screen and the other pointing at the left. Below there's "Garamond", with a "TM" trademark above "Mac". Below the white square there's the text "Starting up..." and the download bar below. The background varies depending on the version; it may be gray or blue (with or without little Finder faces in the background). At the very bottom of the screen, icons representing the system's extensions appear one after another as they are loaded.

Trivia: Extensions were removed in Mac OS X due to extensions sometimes tending to make the system very unstable.

Variants: On every update, there are changes that happen to the logo.
 * Mac OS 7.6 (1997): The "U" in "up" is now uppercase. Plus, the background is now blue with several copies of the Mac OS logo.
 * Mac OS 8.0 (1997):The "TM" symbol is not above "Mac", remaining the background the same as the Mac OS 7.6 update.
 * Mac OS 8.5 (1998): The download bar has a new blue gradient fill. Plus the "Mac OS" wordmarks on each copy of the logo is removed and the background is now darker.
 * Mac OS 8.6 (1999): "OS" is no longer blue and a "8.6" is seen next to the wordmark,thus making it read as "Mac OS 8.6". The background is still the one from the Mac OS 8.5 update.
 * Mac OS 9.0 (1999): The number is changed to "9", thus making it read as "Mac OS 9". The background are the ones from the Mac OS 8.5 and 8.6 updates.
 * Mac OS 9.1 (2001): The number is changed to "9.1", thus making it read as "Mac OS 9.1". The background is now cyan with a cyan gradient fill.
 * Mac OS 9.2 (2001): The number is changed to "9.2", thus making it read as "Mac OS 9.2". The background is plain blue this time.

Technique: The download bar.

Startup Sound (Music/Sounds): None.

Availability: Uncommon. Appears on Macintosh computers running Mac OS 7.6, 8, and 9, such as the PCI Power Macintosh computers and the iconic iMac G3.

4th Screen (March 24, 2001-November 14, 2007)
Screen:
 * 2001 (10.0): On a blue background, a rectangle in the center with lines appears, with a metalic Apple logo near the top, and the text "Garamond" below it. The progress bar is seen below. "Welcome to Macintosh." is seen below the bar. Then, verbose information about the startup appears in place of "Welcome to Macintosh".
 * 2001-2003 (10.1-10.2): Same concept, but with a blue, glassy Apple logo.
 * 2003-2007: (10.3-10.4): It's the same concept as before, but the Apple logo is in white with a streak, "Mac OS X" is seen with a different font, and the loading bar and "Welcome to Macintosh." with theverbose information (in Panther) or "Starting Mac OS X..." (in Tiger, with no verbose info) text are inside a darker rectangle section.

Technique: The download bar.

Startup Sound (Music/Sounds): None (since the "Colored Happy Mac" and the next logo actually serve the startup sound).

Availability: Pretty rare. Appears in Mac OS X 10.0-10.4 computers.

5th Screen (August 23, 2002- )
Screen: A gray background appears. The Apple logo in gray then appears in the center. After a while, a loading circle appears below the logo.

Variant: Starting with OS X Yosemite, a progress bar appears. On newer Macs, the background is black and the Apple logo is white. Prior to OS X Yosemite, on safe boot, the progress bar appeared below the circle.

Technique: The logo appearing.

Startup Sound (Music/Sounds): The 1997 iMac synth chime, again. On newer Macs, beginning with the MacBook Pro, nothing except on macOS Big Sur, where a revamped and low-pitched version of the 1997 iMac synth chime is heard in macOS Big Sur.

Sounds Variants:
 * The revamped Big Sur chime originally was in mono; T2 and Apple Silicon Macs introduced a stereo version.
 * A hi-passed version of the revamped chime appears on the M1 Mini.

Availability: Current. Appears on all the Mac computers using 10.2 Jaguar onwards. This serves as the 1st screen that appears before the 2nd screen of the previous startup on 10.2 Jaguar, and also before the next logo on 10.3 Panther. Versions after 10.4 Tiger has no other screen following afterwards besides the desktop or log-on request.