Auth isn't rocket science - I'd encourage people to recommend well-guided DIY here for newer developers in a lot of cases, because foundational tech learning experiences are critical to growing as a developer.
There's value to many in SaaS offerings, but any decently-sized programming ecosystem has a crap ton of auth offerings. e.g. Devise [0], Dotnet Identity [1], Django Auth [2]. Authorization is the fiddly/annoying part. For authentication, a reasonably motivated developer can expect to have workable, secure password authentication going in a couple hours, as long as they don't try to invent their own encryption scheme.