Clouds

I am a little confused about "clouds"

On my iMac, I have:

Dropbox

ICloud

Google Drive

I would really like to use only one of these products because I don't understand how they behave and it's driving me nuts. Here are my questions (any help would be greatly appreciated)

1. My Icloud drive tells me I'm almost running out of room. However, I don't know how things got there and what is there. How do I stop everything from going there? I would not mind having the ability to store some things there, but not everything. I'm actually surprised how full it is.

2. I kind of like the google drive. But again, i don't understand how things get onto these clouds.

I guess my question is, how do i control what gets lifted up into the clouds and how do I reduce whats up there?

Thank-you for your time and expertise!