Linux Emulation in FreeBSD(docs.freebsd.org) |
Linux Emulation in FreeBSD(docs.freebsd.org) |
The Linux syscalls are just implemented into FreeBSD.
In 1998-1999 they literally used FreeBSD with Linux Abstraction Layer and run Linux apps on FreeBSD instead of using Linux natively because it was faster that way.
Relevant 'press release' here: