PayPal Form: when people pay using my form, where does the money go?

When people pay using my form, where does the money go? I linked my paypal account, but my paypal balance hasn't been changing.

It goes to the PayPal account you set up in  your payment field. Please note that only those submissions where users actually completed the payment at PayPal's checkout page, will generate a payment.

So, if the user submits the form, but does not complete the payment, the submission will land in your "Incomplete payments" dashboard, where you can either delete or complete the submission: https://www.jotform.com/help/135-How-to-View-Incomplete-Payments 

NOTE: Completing a submission will NOT process any payment as there was no payment information to begin with. The system will simply treat this as a regular submission so it can be included on form reports, and that email alerts can be fired off as usual.

We would recommend you to enable PayPal IPN (Instant Payment Notification), which is a POST message sent by Paypal (for regular merchant accounts) whenever a transaction is completed on their side: https://www.jotform.com/help/276-How-to-Enable-IPN-on-your-Paypal-Account 

Let us know if you have more questions, we will be glad to assist you.