Apple

If you just want to run emulators on it you may want to consider installing linux on it. Considering the fact you only use windows the learning curve shouldn't be any worse than OSX and you can install a newer version. You could also install a dedicated emulation distro like lakka, but then you can only use it for that, it has no desktop.

You also can try to install some minimal Linux distro and run some Pihole, Plex, Jellyfin, homeassistant, etc.

Would that newer operating system even be feasible on such an old system, though?

Not really. Apple is leaning hard into high speed low latency flash memory file storage and modern OS version are just not optimised for the HDD in your iMac. It'll be a little better if you install an SSD... but compared to a modern Mac it's still going to be about 10x slower in terms of bandwidth and even worse than that in terms of latency compared to a modern Mac.

The latency is the real killer - when Apple switched form Intel CPUs to Apple Silicon CPUs, a lot of disk benchmark tools started showing "zero" or sometimes even negative values for file access latency, because it was so fast they weren't even able to get an accurate measurement. Apple has optimised their software to pretty much require that speed.

Your 2011 Mac has a disk controller chipset on the hard drive, which talks to the CPU over a SATA interface that was barely fast enough for a spinning metal hard disk drive. Modern Macs have the disk controller inside the CPU. And PCI connections to flash memory chips right next to the CPU (almost part of the CPU).

You're better off sticking to older versions of MacOS which had more caches in RAM than the newer versions. Or, you could put Linux on it.

You haven't wasted your money at only $15. But you also haven't really got yourself a proper Mac either.

Plex media center/server would be a good example of what you can do with it. As people keep saying, you need to drag the app inside of it into the applications folder, the thing showing on your desktop is the image, kind of the exe installer. When you’re done copying the app, drag the thing in your desktop to the trash to ‘eject’ it.

Depending on the actual hardware, 2011 vintage is still enough for the computer to run decently.

You might want to bump up the RAM, but it's young enough that it might still be a decent computer.

I’ve got two 27” 2011 iMacs and I still use them for the kids as browsers/media players/old games. The mobile GPU isn’t amazing, but the i7 version I’ve got plays half-life 2-era games decently. Would definitely recommend sticking an SSD in it if you’re going to use it regularly. Just be aware that the spinning drives have a temp sensor on them that won’t be on an SSD but there are some easy workarounds for it. It’s a great machine for learning (a slightly older) version of Mac OS, or you can install Linux or Windows on it.

For $15, might as well just have some fun with it! If you know linux/unix at all you’ll find the terminal very comfortable, and many of us find that once we’re used to osx it’s hard to go back to windows.

You can install a more modern MacOS with OCLP. Not my main machine, but I installed Monterey on a 2011 iMac without SSD and it runs, though it’s slow, but fine for some purposes.