Installing instruments and avionics in small aircraft

What's the procedure for installing instruments and avionics in small
aircraft?

For example, if you want to install, say, a stereo in your car, it's no big
deal. Lots of stereos seem to be pretty standardized, you just get what you
want and connect a few wires and stuff and its in. How modular and easy is it
in an aircraft?

I presume that when you buy an aircraft new, you get a selection of
instruments on the panel. Is it usually a fixed set, or do you tell the
manufacturer exactly which instruments you want, or what? Can you just
unscrew one instrument and replace it with another? Are connections and
interfaces highly standardized? Do you have to get changes certified or
checked in some way?

Do most pilot/owners have their own eclectic set of instruments, or do most
keep whatever came with the aircraft from the factory? In pictures I've seen
of cockpits it looks like many aircraft are highly individualized, with
various instruments replaced, upgraded, etc. It also looks like at least some
of the mounting and electrical or other connections are standardized to some
degree.