1: First of all, you have to clone or download the project.
2: have 2 version of Angular Cli (for the Angular 5 project you need version 1.6 and for the Angular 10 project you need version> 10) do not do -g !! they must be installed locally not globally.
3: in the Angular project 5: npm install @ angular / cli @ 1.6
4: in the Angular 10 project: npm install @ angular / cli @ latest or npm install @ angular / cli @ 10
5: After the 2 versions installed do npm install in each project to download the nodes modules
6: on the Angular 5 project do: ng serve --port 1200
7: on the Angular 10 project do: ng serve --port 1100
8: normally you will have the 2 projects open simultaneously.
Here ! You will have a buttom in each project to go to the second project and vice versa.
==> I hope I was clear and I am available for any questions.