How do you determine which test case to cover at which level of the testing pyramid?

When determining which test case to cover at which level of the testing pyramid, I consider the following points:

• Unit tests: cover individual functions or methods. They are fast, isolated, and check logic without external dependencies.
• Integration tests: verify the interaction between modules or components, such as working with databases or APIs.
• End-to-end (E2E) tests: simulate user behavior and test the entire system.

I choose the test level based on the goal:

• Logic and algorithms — unit.
• Interaction of components — integration.
• User scenarios — E2E.

For example, I perform data validation checks with a unit test, and a checkout process scenario with an E2E test.