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.
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.
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.
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.
KeepassXC is encrypted locally with no public servers :)