But two site owners decided that they can benefit each other if they provide links to each other's site.

As I said, I am asking my friend to link my Python's and I link his C++.

I can treat him a lunch when I get some ad revenue later. Or just a bro hug.

I understand it may not actually be showing what is happening here, but there is some gray area I want to explore.

I can also be a celebrity and every week I feature a couple links showing what my friends and my other celebrity friends are doing or writing or making. I may or may not get $$. Maybe just to be friendly (think Youtube, how channel owners help other owners...) Will that violate Google's ToS? I am famous and so helping a few people out shouldn't hurt, right?