Can you explain what is meant by the concept of the testing pyramid?

The testing pyramid is a concept in software development that describes the optimal ratio of different types of tests to ensure application quality. It looks like a pyramid with a broad base and a narrow top:

• Unit tests — the base of the pyramid. These are fast and isolated tests that check individual functions or methods. There should be the most of them.
• Integration tests — the middle level. They verify the interaction between modules or components.
• UI/E2E tests — the top of the pyramid. They test the entire application through the user interface. They are slower and more complex to maintain, so there should be fewer of them.

For iOS development, this means writing many unit tests for models and business logic, fewer integration tests for interaction between components, and a minimal number of UI tests to verify key user scenarios.