One way to mass delete is to click/tap on the sender to get the actual email address*, paste that in the mail program’s search, and then select all and delete.

*For instance, let’s say you’re getting Target store ads you don’t want. The email app just says the sender is “Target”. If you click on that it should reveal the actual sender’s email, which might be target@noreply.couponsmailer.com or something. So copy that whole address, pop it in search, and mass delete.

Don’t forget to unsubscribe!!