Apple has done a lot of transitions over its history, and done them with a remarkable degree of backward compatibility.
apple-history.com / specs for every apple computer, established 1996 and lots of Wikipedia articles.
The company started off in 1976 with the Apple I, a circuit board instead of a complete computer. It was succeeded by the Apple II in 1977, and by the Apple III in 1980. The Apple III was not very successful, and Apple discontinued it in 1984. The Apple II was more successful, and Apple made several successors: II+, IIe, IIc, IIgs, IIc+, ending in 1993. Apple introduced its Lisa (Apple IV?) in 1983, and it lasted until 1986.
Apple's first Macintosh (Apple V?) was introduced in 1984, and it was very primitive by present-day standards. It had a Motorola 68000 CPU, 128 kibibytes of memory, and a 512*342 1-bit display. Its OS was very limited: it could only do single-tasking of full-scale apps. Apple soon released a successor with more memory: 512 K.
The last OS version for the 128K was System 3.2/Finder 5.3 in 1986, and for the 512K was System 4.1/Finder 5.5 in 1987.
A curious limitation of the original MacOS was having only a 24-bit memory space, despite it running on a 32-bit CPU. The upper byte was used for its memory-allocation system.
Apple's Mac Plus was introduced in 1986, and its last OS was System 7.5.5 in 1996. That was also true of the Mac II and SE of 1987, the IIx of 1988, and the SE/30, IIcx, IIci, and others of 1989. The Mac II had a Moto 68020 CPU and a color-capable video card, and the IIx had a Moto 68030 CPU.
Apple's Mac IIfx and IIsi were released in 1990, and their last OS was System 7.6.1 in 1997.
Apple's Quadra family was released over 1991 - 1993, it used a Moto 68040 CPU, and some of it could use as much as 256 M RAM. Their last OS was MacOS 8.1 in 1998.
Back to the OS, System 4.1, in 1987 introduced color support in Quickdraw, the graphics layer of the OS.
System 5, also in 1987, introduced the MultiFinder, a feature that let it run several apps at once. But the apps ran with cooperative multitasking, meaning that each app had to give up control to let the others run. That happened more-or-less automatically when the app called an OS function to see what was happening: what keypress, what mouse click or drag, what menu-item selection or button press, etc. So an app could hog the CPU by not calling an OS function. Also, each app had its own partition of the computer's memory, along with the OS stuff; they all lived in one memory space.
System 7, in 1991, had a big jump in possible memory size, from 24-bit memory to 32-bit memory, from 16 M to 4 gibibytes. Apps that could use the full 32 bits of memory space were tagged as "32-bit clean", and they ran alongside older apps.
So MacOS Classic went from single-tasking to multitasking, from 1-bit to full color, and from 24-bit to 32-bit memory addressing.
en.wikipedia.org
