How can I make it so only ppl who paid can use a certain tab in the app?
You can’t prevent any user to navigate through the app tabs, but you can act on the contents of the tabs, using the visibility option for each components of the tabs, or even using per user data.
What about the entire app? Can I make it only accessible if they paid through the Glide App payments function?
I think what you are asking is for a way to allow a person into an app to complete a payment, then open up the rest of the app. You would either have to set up the app as public with email, have a tab with a buy button, and then as @Christophe_HK said, set up a user record in each sheet so you can use per user filtering on the page or visibilty control on each component. Otherwise, you could set the app up as whitelist, then using some kind of third party website, accept registration and payment where a record will be written to the whitelist sheet. Then the user would be allowed to sign in once they are whitelisted.