Yes what is confusing is it might be simpler than one might think.

To run a Linux distribution in jail you need 2 things:

- Enable "Linux Binary Compatibility"

- copy your Linux distribution base filesystem in the chroot or just pick on that is packaged in FreeBSD and do 'pkg install' (Rocky and Ubuntu if I remember correctly)

But you might not even need to "Run a Linux distribution". Just enabling "Linux Binary Compatibility" and executing the binary often works fine if it doesn't depends on a bunch of libraries.

It is really that simple.