Open Gmail
Click the search bar at the top
Type `in:all` to search all mail, including archived messages
Type `-in:sent -in:drafts -in:spam -in:trash` if you want to narrow results to non-folder mail
Search by sender, subject, keyword, or date
Click `More` in the left sidebar
Select `All Mail`
Look for emails without the Inbox label
Open the email to confirm it is archived
To archive an email, select it and click the Archive icon
To find archived mail on mobile, open Gmail and tap the menu
Tap `All mail`
Use the search bar with `in:all` if needed