Let me give a specific example here instead of talking in generalities. I just found this ad below on Pornhub, next to a video (NTV A, top right) but I've also seen this same ad on YouJizz, XHamster and many other tubes:
So graphically (the ad) it's not hard to recreate. The landing page is very similar to the ones I use now and can be altered to look more like this one. So I do not see any special "optimization" for this landing page or join page, nor do I see cross-sells, etc.
Now the one thing I DO wonder is who owns this site. I am assuming it is one of the larger companies so in that case, maybe this site isn't "profitable" at all, but rather simply a marketing campaign to drive type-ins, SEO, affiliate interest, etc. Maybe the Goal for this site is not "profit" from the actual clicks on the ad. If that's the case then we can forget all about this idea. LOL