They work by giving you a random email address that is available for a short time (ex. 10min) and an inbox for any messages you recieve during that time (ex. an email verification message from a Lemmy server)
I didn’t want to recommend a specific one, however Googling “temporary email” should give you several options to choose from.
You would be surprised.  I work at a IT helpdesk at a university and I think 90% of all students don’t know what password manager is. Many have them “stored” in the notes in the smartphone.