I’m kind of curious as to what the person is seeing, because every third post I see is an ad placed by Instagram, and for some of the meme pages you reach nearly that much saturation with sponsored posts.

I mean obviously there are ads, but if my friends post something, it's at the top of my feed, and I can scroll through only content that my friends post (and ads).

It's still not just a linear feed, though. Years ago it was just a reverse-chronological feed of things your friends post. Now the ordering is "algorithmic". I can't think of any good reason to reorder posts other than to try to increase engagement, which is the problem we're talking about here.