A twelve string mixes 'em up a bit, pairing plain and wound side by side for the middle three pairs:
The low E pair has two wound strings, and the high E and B are all plain steel.
A classical guitar may look like it has both metal and nylon strings:
But we say it's a nylon strung instrument, because the three wound bass strings have nylon filament cores.
A five sorting banjo usually has four steel strings and one wound string:
The two bass pairs of strings on this mandolin are wound, where the two treble pairs are plain steel:
While this mandola has only one pair of plain steel strings:
This tiple really mixes up the strings:
From the left, wound, plain, plain, wound, plain, plain, wound, plain, plain, plain. It's just a matter of accommodating the tension and tuning.
Ukuleles have four plain nylon strings:
These are black colored nylon.
Not to worry, there won't be a test. I just wanted to give you an idea of the diversity of uses.