The most legit ones use their own users (to access the residential proxy you have to agree to become an endpoint yourself). More shady but probably legal ones

The most legit ones use their own users (to access the residential proxy you have to agree to become an endpoint yourself). More shady but probably legal ones use JavaScript and Apps that typically hide the fact you are becoming an endpoint in the T&Vs. Straight up illegal ones use compromised computers and routers like you'd think.