Columbia University has released a stand-alone application to run MacOS 9.0.4 on an Intel or Apple Silicon Macintosh. You must have macOS 10.12 (“Sierra”) or later. Yes, to run an operating system released in 1999, you must have a Mac and Mac operating system from at least the year 2016.
http://www.columbia.edu/~em36/macos9osx.html
And yes, it is an application: dump it in your Applications folder and run it like any other application. You are emulating a PowerPC-based Macintosh from the year 2000 with 512 MB of memory, which is a lot of memory for that time. It comes with a nice variety of included programs, ranging from ClarisWorks (a combination word processing, spreadsheet, and database program) to MacPerl, a version of the Perl programming language. A much, much more recent version of Perl ships with every version of macOS X, but back in 1999, MacPerl was a language mostly used by hackers and scientists, and it definitely was not shipped as a standard MacOS component.
MacOS 9 also had Desk Accessories, which were small programs that you could run while you were running other applications. (Multi-tasking, which we take for granted today, was a relatively new concept.) At one time, the average Mac user had dozens of desk accessories, ranging from games to jokes to desk accessories you used to pretend you were working in your office when the boss drifted by, but the emulation has a fairly modest, and saner, selection.
Two web browsers are included, the first being iCab 3.05 (a shareware web browser from Germany, still maintained and supported today; you can download it from https://www.icab.de/). While this ancient browser technically does work, almost every page visited resulted in an error.
The second browser is Classilla, a spinoff of Mozilla, one of the first web browsers. Classilla easily managed to load Google,
However, almost every other web address returned an error message. As an anti-hacker and security measure, Google and Apple sponsored a campaign to encrypt websites, and the encryption method used today simply did not exist in 1999. Attempting to visit the Strait Macintosh User Group site, for example, returned an error.
Classilla may have faded away, but its parent, Mozilla, still exists — Mozilla https://www.mozilla.org/ is the publisher of Firefox, which today has about 2.5% of the worldwide browser market. In contrast, Safari has about 21% of the market.
Is this emulation of a 1999 computer operating system useful in 2023? For most people, no. It is a splendid example of computer emulation, white-hat hacking, and other computer science achievements. It is a fun toy and a nostalgic toy. But it isn’t particularly useful.
And yet, it is a whole lot of fun, if you like playing with antique computer technology.