What's the best way for a startup to store and manage user accounts?

I've written two sites in plain HTML/JS hosted on GH Pages to which I want to add users and eventually charge money for subscriptions However when I google "adding users to your site/startup" I cant find any articles on platforms or services that manage the IT aspect of managing users info and storing passwords securely, etc. So I want to ask you guys what do startups usually use for setting up their users account and related management software?

Asked another way, if I get a server on Netlify, Heroku, etc what framework could get me started with built in secure authentication?

Thanks this is exactly what I was looking for. Found out it's called identity management on AWS as well as other services like Auth0 and Okta.