Imported accounts are those imported into your wallet using a private key string or JSON file. They are not derived from your MetaMask Secret Recovery Phrase.
For this reason, these accounts will not appear automatically when you restore your MetaMask wallet with your Secret Recovery Phrase. The data associated with your MetaMask Secret Recovery Phrase cannot be added or extended to the imported account, and importing an account previously does not associate it in any way with the Secret Recovery Phrase associated with your wallet.
Just as you'd keep your MetaMask Secret Recovery Phrase securely recorded, you must keep imported accounts' private keys safe to ensure you can recover/access them in future.
Imported accounts are labelled in MetaMask's account selector dropdown menu, which you can find centrally at the top of the homepage on Extension, and at the top of wallet view on Mobile:
For information on how to import accounts, see here.