I'll say it again. Linux roots are not in Unix but was an attempt to copy Unix.
Freebsd roots are in Unix because it was a source copy of Unix
We are quibbling here about exactly what constitutes Unix. If I were to define it as "that which was written by the Bell Labs research group headed by Dennis and Ken and first published as Unix", then your first statement is true (no lines of their copyrighted code is in Linux, and never has been), and your second statement is false (no line of their copyrighted code is in BSD today, all the AT&T lines were removed long ago). We could instead define it in a different fashion. But the OP asked not about source code, nor about kernel development.
At a user level, what a normal user sees from a CLI is utilities and programs (such as cp, rm, bash, tcsh, vim and emacs) that are extremely similar or identical between the two OSes. From the GUI, they see an environment like Gnome. And if the OP does user-space programming, nearly all the APIs that they will use (simple ones like open() and read(), and obscure ones like tcsetattr() or pthread_cond_timedwait()) are nearly the same. That's because they are either identical and built from the same source (bash, emacs, Gnome), or their implementation conforms to a standard, usually POSIX.
That was my message above for the OP: Don't be afraid to learn about Linux, because much of what you learn there will also apply to FreeBSD. But not all will.
Where they differ are (a) Minor corner cases in common areas, like rarely used options to commands. (b) Which software is installed by default; for example BSD ships with its own make, Linux ships with GNU make, and while compatible at a basic level, the idiomatic style of makefiles is different between them. (c) Kernel programming, as you pointed out. (d) Some areas of system administration. You pointed out the example of systemd, which is the same example I used of how to create services. But mostly, things are the same, in particular for a beginner.
