Archive

Assuming that you are using an IMAP mailbox, if using default settings, you’ll have both a local copy in the corresponding mail folder on your iPad - and a copy similarly saved on your email providers mail server. By contrast, if using a POP3 mailbox, depending upon configuration, you might only have a local copy.

For each configured email account, the Archive (and other folders) a configured here:

iPadOS14

Settings > Mail > Accounts > [email Account] > [Account] > Advanced > [Mailbox Behaviours] > Drafts | Sent | Deleted | Archive

iPadOS13 and earlier

Settings > Mail > Passwords & Accounts > [email Account] > [Account] > Advanced > [Mailbox Behaviours] > Drafts | Sent | Deleted | Archive

You are very welcome. I’m delighted to have been able help.