Well it goes onto a raspberry pi which acts as a web server, that connects to the printer. You then visit the octoprint website interface to manage connected printers. That's handy because I can do it from my phone or computer so long as I am on the wifi, and you can in theory reach it remotely while out of the house if you set up your network to allow that.

It works well on a raspberry pi but there are multiple ways to host it (docker, etc) so even if you have old spare hardware you're in luck.

Not at all in my anecdotal experience! My Prusa Mini just connected to my old Pi via MicroUSB and it all "just worked" for me. Ended up adding a Pi camera to the setup and installed the OctoLapse plugin to automatically generate timelapse videos of my prints; been very happy with it all so far.

Another data point:

I also found it super easy. The most difficult part for me was finding a data capable MicroUSB cable to connect the Pi to my printer. Most of mine were only for charging. I used an old RPi 3 and used the OctoPrint SD image. After that, everything just worked. It even worked with my USB webcam instead of the default RPi camera.

This was connected to a Creality Ender 3.

It might be more difficult to install as a package as opposed to using a pre-made RPi image, but it should still be pretty straightforward.