Linus T explicitly licensed Linux under a license that allows anyone to run it but requires people who modify modifications to share those modifications.
Correct! This is the exact reason anyone who wants to use the os itself as a moat uses FreeBSD as a base instead, and add proprietary modifications to it. FreeBSD also being a open source gift, that does not have those requirements that Linux does.
Prominent examples include Sony PlayStation, and Apple OSX.
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> but requires people who modify modifications to share those modifications.
Not exactly. You can modify Linux and run it yourself all you want without obligation to share your changes. The sharing requirements are more limited and involve distribution.