I believe yes, a c compiler is part of the unix specification. But linux by
itself isnt unix, its unix when you add on a lot of gnu tools. There are
plenty of good use cases for an operating system with out a compiler, often
in the realm of limitation. Personally I like BSD systems due to them being
complete unixes maintained and released as one piece(typing this post from
FreeBSD). 

Best way to learn and understand your tool set? In my opinion, dont open
google, dont read an orielly book, stick to man or info and provided
handbooks- for everything. Sorta forces your brain to understand the
information in the provided context.

--- Mystic BBS v1.12 A43 2019/03/03 (Windows/32)