Branches & CLONES in GITHUB - where are they stored?

Hi all. I'm a noob coder taking tiny steps into GITHUB. I understand how to FORK, that is as easy as pushing the fork button.

However, when I'm making changes to the "FORK", does a BRANCH automatically get created? Do you automatically create a branch when you've "FORKED" the main repo? If a branch is automatically created, where is that stored? On GITHUB? Or do you have to store your FORK-->BRANCH on your local pc? Is the clone done on the "BRANCH" or the "FORK"?
Sorry for the basic questions, but I want to get good at GITHUB.

Thx,