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.
I’m personally deeply skeptical, even if it’s from one of the most secure companies I would have a heart attack if I hear it’s hacked.
I personally just use pen and paper, two copies, and try to do variations or new passwords for things I care about and then one common password for useless things with no credit card info.