Could you give examples of unit tests related to the entity inside the question?

Question

Software testing

For example, there is a certain implemented service that returns us a list of meetings

Tests (API):
1. Calling the service, that it basically works, returns something
2. Checking request parameters
2. 1 valid values
​​2.2 invalid values
​​2.3 do not specify the obligation parameters
3. Checking the return of a specific number of meetings from the database
4. Checking the return of an empty array of meetings from the database

Obviously, these are functional tests.

And what would unit tests look like for such functionality?
Do I understand correctly that they would test some pieces of the internal implementation of the service for returning a list of meetings? And with my tests, I’m already testing the resulting product (in my case, a service), as it were, at the top level, do I understand correctly?