I'm back to Windows 10 (now 11) on my main PC since I bought an Xbox and there's hassle-free Cloud gaming, crossplay etc.
When I exclusively played on PC and built the new Machine, I was too cheap to buy a Windows licence. I tried Pop!OS because I like their gaming-focussed apporach. All games that were relevant to me (via Steam, mostly) worked fine.
I've since bought a Steam Deck, so I'm running SteamOS as well.
I have never once paid for Windows, yet at some point a digital license showed up on my Microsoft account and I've never gotten an Activate Windows prompt on any of my subsequent Windows installations.
I don't think so, I've swapped motherboards several times since and the license has remained.
I used to borrow Windows keys off the old PCs in my office's e-waste pile, my best guess is it's one of those keys that's somehow still activating each time I reinstall.
There are utilities like MAS (open-source project on Github ironically) that activate Windows fully without anyone ever needing to pay for it, just in case anyone looking at this thread was wondering
It's actually comically easy to get Windows on a machine because Microsoft wants everyone's default OS to be windows as much as possible. Its an easy loophole to use Windows without paying for it at all, ever.
I can't really say. It has a desktop component but it's not the main focus. I've seen setups of people who only use the Deck as their PC, but they mostly don't do much aside from browsing the internet in desktop mode (Firefox is preinstalled). It's pretty nice that you can plug it into a USB-C Dock and have it connect to screens, keyboard and mouse.
I'm no expert on customizing linux, but you probably can get some usual features to work on SteamOS.