Mailbox not merging

Open the Mail App sidebar. Scroll to the top - and you should see the All Inboxes and other aggregate mailboxes.

You can control the available mailbox views. If the All Mailboxes view isn’t visible, tap the Edit button at top-right of the sidebar to expose the extended mailbox list. Individual mailbox views can be enabled (ticked) and disabled (unticked) - and rearranged by dragging the associated “hamburger” icons up and down the list.

Tap Done when you’re finished editing the sidebar.

The iPad User Guide provides more information. Here is a direct link to the relevant section of the Guide:

Organize email in mailboxes on iPad - Apple Support