OEM licenses for windows 11/10 are dirt cheap even here in the U.S. You can get a single use (can only activate once) for around $10-15. Most of those online sellers are based in China or HK. So yeah, those are legit OEM licenses. Since MS doesn't seem to have issues activating them. (I've bought them for VM use before)
i am unfamiliar with windows activation, does it activate once only, meaning that if you tried to reinstall on the same machine, you can't reuse the same license?
Correct. If you need to do a fresh install on the same or new drive, then you'll need a new key. I only use these cheap OEM keys for VMs and use more expensive retail keys that persist with MS login on physical hardware. Although I know plenty of people that build own gaming machines with OEM keys.