Seeding database data for tests

I’m a bit confused as to how Ecto works with seeded data when it is conducting its tests. What EXACTLY is the flow when you commence a test run (whether it be all of them via “mix test”, or only a specific file, or a specific test by specifying a line number)? Does the testing database get seeded in this process?

For example, I have some data (e.g. a list of countries) that should be the same in all environments. Do I need to explicitly seed that in the testing database? I have other data that I COULD seed ONLY in the test database just to help write thorough integration tests. How should that be best accomplished?

Thanks for any guidance!