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.
Are there really people out there not using a password manager in current year?
Yes, my dad, I created an account for him on my Vaultwarden instance, and hardly doubt he gets to use it.
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.
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.
When I first set up my password DB I forgot my master password. Fortunately that didn’t happen on my second. I still have the locked first db.
Many people don’t like putting their eggs in one basket. Which I get. But imo as long as you’re properly securing everything as well as you can, most decent passwords managers are trustworthy enough.