Beginner questions about Brandclub

I just got started with Brandclub and have a few questions. I'm guessing they are already addressed somewhere; if so, I'm hoping you can point me to a reference.

1. Retailers Included: Does Brandclub only show results for linked accounts? If not, how do I see other results?
2. Linkable Accounts: I see Walmart, Target, Kroger, Chewy, and Amazon as the linkable options. Is this the list for everyone? Is there a way to get more or fewer (other than Brandclub adding or removing some later)?
3. I can join clubs in Brandclub. Is there a way to leave one?

I tried to research this myself but I couldn't figure it out. For #1, I see a lot of reviews saying "link your accounts, like Walmart, and then do bla," but I haven't seen things saying "you can't do anything without linking."