People build botnets by enticing people to install trojans on their computers, e.g. a free utility app or game.

They can then earn money from people who want to rent access to these botnets.

There are also free VPN services who, in their fine print, say that users grant them permission to route other traffic via their connections.

There are "residential proxy services" offering exactly this and you only ever pay for bandwidth. Using 100,000 unique non-datacenter IPs will only cost you few thousand dollars as long as you only sending tiny API requests.

And this is service offered by registered Israeli company that get formal agreement from "bots" to route traffic through them. Very shady, but totally legal service that used by a lot of data collection agencies for price tracking on Amazon or getting data from Linkedin, etc.

botnets. With all of the crappy IoT devices out there it is even easier to get inside of consumers networks.