alt Hacker NewsDisagree on the method:
I recall that bot farms use pre-paid SIM cards for their data connections so that their traffic comes from a good residential ASN.
No client compromise required, it's a networking abuse that gives you good reputation of you use mobile data.
But yes, selling botnets made of compromised devices is also a thing.
SIM cards is (one) of the ways the big boys do it. It gives you a nice CGNAT to hide behind and essentially can’t be blocked without blocking a nontrivial chunk of the country. Although more and more fixed-line ISPs are moving to CGNAT too so you can get that advantage there as well.