I'm accessing my Gmail account via imap with Thunderbird.
When I move a mail to the imap spam folder, that action equivalent to using the Report Spam button inside Gmail's interface? Does Gmail's spam filter learn from this action or not?
I was wondering the same thing myself. The answer is Yes.
Taken from this Google Help Article
Using the button in Gmail and the imap spam folder from Gmail is an equivalent action.
Junk mail and spam:
In my experience, using Apple's mail client with Gmail (imap), there will be a spam folder specific along with the other labels, synced as folders. That's the folder you should be using - not anything that's Thunderbird specific.