I started getting into lots of good content on youtube about a year or two ago. Before that, EEVblog was really the only thing I subscribed to.
After having watched a lot of youtube, I have seen a ton of different ways of monetizing channels and cross promoting channels.
Some of the monetizing stuff is subscription based, like patreon (or subbable), and often, Audible.com sponsors channels.
I've also seen a ton of collaborations. There are interchannel collaborations, and company / channel collaborations.
For example, General Electric did a collaboration with The Slow Mo Guys. The slow mo guys went to some of GE's facilities, and filmed some tech GE was working on in slow motion. One of the things was a superhyrdophobic (water repelling) coating, and how the water pooled and cleared off in slow mo. The content fit the channel (cool stuff in slow mo), and also promoted GE. Another couple examples are colinfurze and both HTC and some videogame (assassin's creed maybe?). HTC sent him a bunch of phones and asked him to do something unique with them. He tested the phones for durability, and sent them to the edge of space in a weather balloon and tried to shoot off a firework up there. For the videogame, he hot-rodded a tuk-tuk, and added bb guns, to resemble a weaponized tuk-tuk in the game. All of the content fit the channels they were on, but also happened to be promoting a product or company. It was definitely product placement, but I'd classify it as a new era of product placement, where it's not obtrusive and irrelevant.
I've also seen a few channel collaborations. Sometimes one channel I watch will collaborate with another one I watch (that's always fun). Sometimes they will collaborate with another channel that I don't watch. If it's something I'm interested in, I will probably go check out the channel. Often, there are two part videos, with one part being on each channel. I can't think of a lot of examples of this, but one is MKBHD (a tech reviewer) recently did a top electronics of 2014 video of something, and collaborated with another channel for a section on phone cases (MKBHD doesn't use cases, so this collaboration fit very well: solid reviews of a couple phone cases from someone who uses them a lot, but still relevant to MKBHD's viewers).
I've also seen collaborations that didn't make a whole lot of sense. Imagine EEVblog collaborating with a makeup channel, maybe by doing a teardown of some laser hair removal product. Not really too out of place for the EEVblog, but that collaboration would be a little strange.
Also remember that even you have done at least on collaboration: That apollo LVDC board from Fran. Think you got any new subscribers that were referred from her channel? You've also mentioned mike's electric stuff channel a couple times as well.z
I have no idea if CMS are behind all of these collaborations, but I'm sure they're behind some of them. Not sure if they're worth it at all, but collaborations with other youtubers definitely happen, and do expand their user bases.