No one wants to tell you because that's how we make money .
There are botnet ones out there(if I had to guess, Rsocks and other russian based ones). But there are more legitimate networks out there that have other means of getting many, many IP addresses.
They release free vpn with 10 servers, advertise it over the internet, get a lot of vpn users, vpn client has hidden feature to make residential proxy out of pc. As simple as that. I also think that most sellers are just resellers of bigger providers.
There are many options but these are the ones which are most profitable:
1.You provide free vpn via app or a company and get their Residential Ip in return.
2.You make a botnet and spread it massively which is illegal but the most profitable one.
Crime is money.
I ment in a general, nebulous way.try that here and you wont last long