I wonder how much these smut writers get paid

Do it!

Not a lot. Go try your hand at Literotica, it's a good place to get some practice. A lot of the writers I know from there self publish on Amazon.

By all means get writing.

Self publish on Amazon KDP. If your stuff is good, editorial directors will try to get in contact with you provided you listed an email in your author bio. From there you secure a literary agent and you're on the road to getting published.

Someone I know self-publishes on Amazon.
By giving them exclusive rights she gets to keep most (or perhaps all) of the sales income.

I downloaded the kindle app onto my laptop to read one of her books.

I think Smashwords lets you self publish, some asstr (defunct erotica site) authors have published there as well

If you go the electronic self publishing route, please share your pen name. You can write quality stuff.