This is probably not the best place to ask this question, but as a solo founder or just to reduce costs/time are there some standard free software packages that are used when creating sites? For example most sites need a user sign up mechaism, a authN and authX mechanism to gate access to different pages. Are there open source projects that provide this? Or do site owners develop these from scratch every time?

Different framework implementations of a CRUD website with authentication.

A lot of popular web frameworks have basic authentication out of the box & easily allow you to tie free authentication with accounts like Google, Microsoft, and many others. There are also paid alternatives that may save you more money than the free ones if you need advanced authorization controls or other features.

Most devs probably have a collection of ways they've done it in the past that they pull from when needing to adjust from the default framework's methods.