Outside of free software/shareware, there wasn't a lot of software available for download in the 90s. And compatibility was a lot jankier in that the base systems sold to consumers were insufficient to run a lot of popular software. For folks like you or me, probably not a problem (unless you happened to have a graphics card that wasn't supported) since we'd generally have obscene (by the standards of the day) amounts of hard disk and RAM, but Mom's computer might have 4M of RAM and a 10G hard drive.
Back in the floppy disk era the thing that always drove me nuts was that most (with very few exceptions) software vendors put the software key on the label of disk 1. Which would be in the drive when they asked you to enter the software key. I remember some OS/2 software that put it on disk 2 instead and I wanted to go to the company and kiss whoever it was who decided to do that.
Well - as recently as a couple years ago - even if it was a web-based tool, you would have to check which browser was supported.
Things have gotten better now that Microsoft has deprecated the entire IE-chain and moved to Chromium-based Edge. Things "just work".
You still have to today. A lot of stuff is Mac/Windows/Linux exclusive. Sure, I can run Windows stuff on Linux and vice versa if I'm tech savvy, but Mac stuff requires buying Apple hardware, so that's a massive barrier right there.
>figure out where and how you can buy it
Or you could just download it. The internet existed pre 200x as well.
> then manually enter a product key (fun!)
Honestly, it was fun. Much more fun than the phone-home, always-on telemetry and invasive DRM that act like rootkits & spyware we have today. Good luck playing older games you bought and paid for who's DRM require authentication from the publisher's servers that went offline years ago. Yeah, fun!
Back then, you paid for the product, you got your license and CD key, you owned it forever. No subscription rent seeking model BS.
Maybe I'm just old and cranky.