Before Apple released their new and shiny Mac OS X (Mac OS 10) we used to use other versions of Mac OS which is called Mac OS Classic now.
The whole architecture and the design was quite different but you might remember them if you used any PowerPC based or older (68k) Apple Macintosh computer.
The last version of Mac OS Classic was Mac OS 9.2.2 and after that Apple introduced Mac OS X and then they introduced macOS.
If you want to run MacOS Classic you either need to have an old Apple Macintosh computer or you need to run emulators like Basilisk II and SheepShaver but they are limited.
Basilisk II runs MacOS 7.x, 8.0 and 8.1 depending on the ROM being used and SheepShaver runs MacOS 7.5.2 thru 9.0.4
There are few other emulators which makes you able to run Mac OS Classic 9.1 and 9.2 and they are based on QEMU emulator.
QEMU is a generic and open source machine emulator and virtualizer and you can use it for emulate almost any sort of machine and install almost any operating system.
You can install and run old applications and enjoy the feel of the Mac OS Classic. it’s so nostalgic!