How to import another "app" into my next-project?

Question

I am building my very first react/next.js-page.

Time is short and I would like to cut some corners if possible.

You can find it on the master-branch of this repo: https://github.com/ehak123/SpeedDating

I want to add a simple login/registration-form and found this great "how-to-guide" online. It really looks sleak. The code for the finished login/reg-form is found on this repo: https://github.com/cornflourblue/next-js-11-registration-login-example

How do I in a simple way import this into my own project?

I have tried to "merge" the two projects in so many ways but keep hitting into problems. I searched the web for help, but cant really find any. I figure: it must exist an easier way, like a custom way to do this?