The following manual describes the way to get your signature to the default Mail app on iOS for 14.0 and newer versions. If you're looking for the Gmail manual, please follow here.
Go to the User page on your iPhone or iPad
Scroll down to the Download HTML option
3. Click on it and you'll see a prompt to download the signature. Confirm it.
If this option didn't show for you but your signature popped up instead, go back and Tap and hold the orange Download HTML link. A menu will appear, choose Download linked file
4. Go to Files
5. Tap and Hold on the signature file until a menu appears:
6. Select Copy.
7. Now launch Settings from your Home screen.
8. Go to Mail.
9. Scroll all the way down to Signature
10. Delete your current signature
11. Tap and hold for a second, when you release your finger, an option to Paste will appear.
Your signature is now ready and will now appear at the bottom of emails sent from your iPhone or iPad.