Technically volume and gain and compression are separate things but they are related. Mixing process is generally where you adjust volume or gain on individual tracks and you can apply compression on individual tracks as well. Compression generally acts as “glue” during the mastering process to bring levels of individual tracks closers together by applying it to the master track. Very high level explanation.

All answers?

Ok. Here goes. The short answer is it depends on the arrangement. Big stars have more clout with publishers/producers, many artists self publish and maintain control themselves, other work with artist centric studios who respect artists wishes. Otherwise, it's usually the studio who have a say, and they rely on their mastering engineer's opinion.

The trend for the last long while was the "wall of sound" pionered by Phil Specter who developed a compression technique so it would sound good on cheap systems and especially cars and radio as that was the main means to introduce music to potential buyers. It has persisted into the age of streaming. Hi-fi friendly masterings are usually done because the buyers are into that kind of music and seek it, conversely, most pop is mediocre. The short of it is the loud and quiet parts are evened out so you can hear more over road noise.

I recall one example, Mary Star of the Sea by Zwan (Billy Corgan from smashing Pumpkins) who made a fantastic record, but wanted, as an artistic choice, for the album to sound like it was being played on a boombox. Great music, sounds like shit. I consider it one of the greatest mastering crimes of all time.

A lot of it is also in the recordings in isolation booths so you get a perfect individual performance, but you lose the stereo imaging and staging audiophiles love. Really depends on who is doing it and why. A lot of artists don't know anything about this stuff and count on the pros to take care of it for them. Its pretty diverse overall.