Same in mine, spammers buy blocks of numbers (for example all numbers from XXXXX100 to XXXXX999) and use them to avoid being blocked by people who block individual phone numbers (with the default phone app). Fortunately I found Yet Another Call Blocker on F-Droid and I can block by pattern with it (XXXXX*) but most less technical people won't be doing that.

You don't even need that, at least in America. Phone spoofing is terrifyingly easy.

Do you not have to pay a network provider in order to use it to make calls and send messages? How does this work?

I don't think that's true. For one, the sign up of the accounts itself is often automated and in the hundreds, because bot accounts are squashed all the time. You'd spend thousands on burners.

Spam is mostly prevalent because it's cheap and easy to throw millions of requests at the wall and see what sticks. If it did cost really any money at all it wouldn't be remotely worth it.

Spammers don't just use one account to spam.