They use virtual credit cards (that have no balance on them) to create accounts with Spotify Family Plan.
They don't pay, they use the trial which is 1 month free.
The family plan has like 6 spots where you can add 6 different accounts by sending an invite link to them.
They login to the customer's account from an incognito browser, paste the invite link from the family plan account and accept the invitation to join the plan.
It's simple but that's just one variation of the process, there's different ways to go about this and even tools to automate this process.
With netflix idk if there are family plans, it's the same process but they just create 1 account at a time and sell it.