Once user pays with Paypal, are they redirected back to the form?

No, the user will be redirected to the Paypal confirmation page. In that page, there is a return link wherein if the user click it, he/she will be redirected to the Thank You page  you created in JotForm.

If you want to redirect the user back to the form, you may enable the "Auto Return" function in PayPal. Here's a guide: https://www.paypal.com/ph/cgi-bin/webscr?cmd=p/mer/express_return_summary-outside. The URL that you'll enter in the Auto Return is the form's direct link.

Hope that helps. Thank you.