I was pretty sure that I got something via gog and transferred the license to steam. It might have been another service.

Gog is kind of exempt from the licensing complaints since they're DRM-free AFAIK. So you can save, back up, transfer, and otherwise hold onto any of the games you purchase. They're doing good work regardless.

I know other services let you transfer keys around. With steam you can always see/access your product keys. So that's a good thing. I know with epic, you can export the keys for steam and other platforms I think.... Which is okay.

I wouldn't know how anymore. I haven't purchased a game that wasn't on Steam for so long that I just can't recall the process to do it for any platform.