When you see an email in your spam folder that is not spam, swipe and move the email to the inbox or choose 'Not spam' in the swipe actions. This will move all emails from the same sender into the inbox. This action will also add the sender 'From' address to the 'Add to Spam Safe List' so their emails won't be marked as spam in the future.

You can also tap on the 'From address in an email to open the email detail screen. In the bottom of the contact screen, tap on 'Spam Status' and the status will show if the sender is blocked. Then in the spam screen you can tap on the 'Add to Spam Safe List' button to mark an address as being valid and never mark it as Spam. In the bottom of that screen you can do the same for the email address domain.

The 'Not Spam' rule will also detect emails in the Spam folder that are not spam and move them to the Inbox.
Make sure this rule is active in the email rule screen More details