If you're not using forms from a model driven app/canvas app to update your data then the below might not be relevant to you.
Lookup:
If you add a Lookup field from a related table into a Main Form, the user may have limited options in terms of what they can see in the Lookup.
In the below example, after the user clicks on the magnifying glass icon, they will see a list of records available in the lookup field. By default, we get the ID and the Created date field, but you can modify what fields are displayed in this initial view, as well as what gets displayed when the user clicks the Advanced lookup button.
To modify this, navigate to the related table, select Views and then edit the Lookup View. This is the View which is displayed in the search box in the model driven app.
Quick Form View:
A quick form view is a read-only view which can display data from a related table row inside your Main Form. This is useful because it allows you to quickly view related information without having to navigate to a different screen or different app.
Navigate to the related table and select Forms > New Form > Quick View Form (remember this is a Form, not a View).
Make your customisations to this View, then Save and Publish. Then go back to the Main Form in your App, select Quick view from the Components tab, and then select the Quick View form.
For more detail, @lisacrosbie has a wonderful video breaking this down:
https://www.youtube.com/watch?v=TU9VtY4LZLk&t=179s
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