Leftover processes after closing programs leading to gobbling up of RAM - how properly kill all processes related to a program when closing?

I'm running Bodhi Linux.

When I open Chromium, browse a bit, then close it, the RAM stays about 1Gb more occupied that it was before. This is the same if I start running other things, even installing things in terminal or running and closing zoom. It just doesn't shut down all the processes it was using.

Everything eventually creeps up to nearly 75% use of my available RAM doing nothing.

How do I close a program and actually kill everything associated with it? I have been trying with htop, but searching for the names of the programs I can't see any of those programs left. I don't want to randomly start killing processes as this would be bad I'm sure.

Can you advise? Thanks