FreeBSD has supported Linux emulation for a long time (https://docs.freebsd.org/en/books/handbook/linuxemu/). The emulation is at the syscall level but enough to run most apps.

There is a significant investment in a linux compatible system call layer, and a linux compatible runtime link library suite.

It isn't a complete answer, but the position as I understand it (haven't had to care for a long time) is that a LOT of linux binaries can work.

> can you run any Linux distribution in a FreeBSD jail

-ish. There's a compatibility layer that works at the libc level but not the syscall level. In practice anything open source that works in emulation almost certainly has a FreeBSD version and anything proprietary that actually needs Linux will work better in a VM.

If I understand the question correctly: You want to run a Linux distribution like (say) Debian in a FreeBSD jail? With the Linux kernel and all?

I believe you are asking about bhyve for running Linux on FreeBSD. Jails are for software isolation.