No, but I guess e.g. physics.stackexchange.com or judaism.stackexchange.com benefits a lot from the SE internal links, and I guess for those smaller sites it is the main source of page rank.

That is, if page rank still works like Google claims. I'm pretty convinced that there is a lot of manual curation going on. At least for popular terms and for the first page of results.

Yes, internal links can be extremely valuable especially in narrowly relevant niches like SO.