The first question is, when I exceed the free limit, additional fees are applied, or does the application stop working, if additional fees are calculated without my knowledge, tell me how to determine the reading and writing operations for the free limit only, and when I exceed it, the server stops, for example.

Note: The application that you created can be written by everyone without an account, and the data is visible to everyone as well.

Solution 1: Jan Smitka

When you create new project on firebase, your billing plan is set to "spark", whitch is a start plan. That means, you will not be billed anything, unless you change your billing plan. In this plan, you have 50k reads/writes per day (it was like this a year ago, i am not sure if it wasn't changed). If you exceed this limit, the firebase API's will simply stop responding, so your app won't get any data furthermore that day. If you want to see your billing plan and daily/monthly usage, go to "usage and billing" in your firebase project overview. Here is a small screenshot where to find the setting:

firebase billing overview