I hand-rolled my authn scheme in half a day (backend endpoints only; f/end HTML took a longer). In addition to sign-in with email/password combo, it also does passwordless (i.e. if a user set their profile to password-less, anytime they attempt to sign in it sends a password to their email that is valid for 15m).

It's better (i.e. more secure) than at least two currently-in-use corporate systems I have seen.

And no, I don't store plaintext passwords, and I do use a generated random salt when a password is set, and the crypto functions I use are well-regarded and well-used, not something I wrote myself.

In summary, I don't think it should take two days for a minimal authn/sign-up/sign-in implementation.