This video walks you through how to set up a mono repo using Nx with Angular and NestJs.
The code of this video can be found here: https://github.com/kedevked/angular-nest
For more about poly repo vs mono repo, here is an interesting article of Ben Nadel: https://www.bennadel.com/blog/3944-why-ive-been-merging-microservices-back-into-the-monolith-at-invision.htm.