Archived emails in Gmail are simply emails but with the "Inbox" label removed.
There's several ways to retrieve them.
1. You can either use the search box which will include archived emails in the search results.
2. You can use the "All Mail" option and look for emails without the "inbox" label.
3. You can use Gmail search criteria to find only the archived emails that you're looking for.
Check out this step-by-step guide if you'd like to find out more. The guide explains how to do this using the Gmail website and the mobile app (Android) too, step-by-step and in plain English