What are communities and how do I change which ones I am in?

The easiest thing to change are groups, followed by interests, but I haven’t figured out how to change communities. In addition, why do we have to be in a “minimum” number of each of these types? It’s frustrating and I’d like to leave certain communities but can’t because I “must be in a minimum of five communities”.
Same with the interests- we are forced to choose 10 interests, and the same for groups.
And on a similar topic I’ve always hated when I sign up for a certain service (here, Pinterest, etc) and are forced into joining/following a selection of “popular” groups or topics before I can search the groups I actually want to follow. (And I will immediately leave the forced-upon groups)