I remember the path my license had: it was a free upgrade to Windows 10, from Windows 7 (right before they removed said free upgrade; I tend to be slow with adapter Windows versions). The original Windows 7 license was a pirated one, but that didn't matter (we know why: before GDPR, Microsoft could spy on Windows 10 users, and the pirated Windows 7 was already a lost sale).

Apparently the free upgrade was OEM, bound to the hardware. I did not know. Either way, I'm from Europe (EU), and here a software license cannot be exhausted via second hand market, so it stands to reason I can buy one second hand. That this isn't what Microsoft support is told to discuss, suuure (even when I explicitly asked for it, they insisted I had to buy it via them).