How to become reseller for a brand? Need Help.

So you see “Other Sellers on Amazon” which are some third party sellers selling same branded item as the original seller on Amazon. Now, why would a brand allow that? I know a reselling auth letter is required so these sellers are approved. But given there is FBA, why can’t a brand just send the items to FBA rather than allowing third party sellers? And how many third party sellers are allowed? What if there are too many third party sellers for same item ? Does amazon puts them on rotation? Can one resellers undercut other resellers of that item? Are these sellers only allowed to ship themselves and not allowed to do FBA?

Anything else I should be aware of before I get into it?