Yes, they own what employed workers produce. But especially before there was such a number of freely available open source licenses software vendors licensed to

Yes, they own what employed workers produce. But especially before there was such a number of freely available open source licenses software vendors licensed tons of stuff, often in source, often without permission to relicense the source and over time developers refsctored the licensed code, which makes it hard to trace code back. Especially since version control often was done by having different sets of floppies, which are all gone.