Just a basic question of how to unclog. The water does go down the drain,
albeit very very slowly. I'm sure this is due to an accumulation of
hair/gunk.

I can't figure out how to remove the drain cap. It is the type with a knob
that you rotate about 1/4 turn to collapse it to be able to take a bath, and
a 1/4 turn the other way to keep it raised for showering. It will not
unscrew.

I thought I would first try to unscrew the cap to see if I could access the
clog, but I can't even unscrew the cap.

I don't want to have to call a plumber - I'm sure with the right advice this
has to be a simple thing.

Should I apply a commercial drain clog product? Is there a simple way to
remove the drain cap that I don't know?