We tried to set expectations early on that this tutorial is more focused towards doing than explaining. Now that the doing is done, explanations are in order.
Filling in the gaps on concepts that we applied
We used React components and APIs here with little explanation. Having a solid understanding of React is essential to using Expo to build your app. We recommend reading the Main Concepts section and the Hooks section of the React documentation.
How to verify your learning
You understand how to use React.useState, that it is a hook, and what the equivalent for React class components is.
Add a new button to clear the selected image state.
You can create a reusable Button component to clean up duplication of TouchableOpacity / Text.
You will typically be referred back to app.json as part of guides that explain how to accomplish specific tasks, as such there isn't much you need to learn beyond its existence and how to use specific properties as the need arises.
Topics that we didn't cover and you will soon care about