@Az8PnG To understand how this works, let’s use a basic Form in a Canvas App as an example.
Suppose we have a Table with a list of Issues, and another Table with a list of associated Actions. The Primary Name column in the Issues Table is "Record ID".
In the Actions table, we create a Lookup column associated to the Record ID column in the Issues Table. We will call the Lookup column “Related Issue” (a one-to-many relationship).
1. We create an auto generated three screen app based on the Dataverse Actions Table.

2. We add the Lookup field into the Edit Screen of the Form if it not there already. You will notice Power Apps will automatically create a Combo Box control for the Data Card Value.
3. Note the Choices expression used in the Items property of the Data Card Value, it is using our relationship Lookup column ("Related Issue").

4. If you select Edit on the Combo Box properties pane on Fields, you will notice you can select any related field from the Issues table.

5. Suppose we want to include a Single Line Text column from the Issues table. Add a custom card and include a Label control inside it.

6. In the Text property of the Label control, enter ThisItem.'Your Lookup Column Name' followed by a period/full stop.

7. At the period sign, notice you can now select any column from the related Issues table
8. Final result:

If you're using Model Driven Apps, you can achieve similar functionality using the Related tab or via Views such as the Quick View Form.
https://learn.microsoft.com/en-us/power-apps/maker/model-driven-apps/types-forms
In summary then, so long as you have defined a relationship between your two tables (such as a one-to-many relationship), you will be able to return any associated column from that relationship in your App.
I hope this helps.
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Imran-Ami Khan