Internal Projects are exactly the same as any other Project, however they are accomplished for/on behalf of an internal User/Team. In this way, you're doing projects for your own company instead of an external client.

Internal Clients will be available in any given client list. Your Client list will display the External clients first, and then show each of your Teams with the relevant individual users seen underneath:

Once an internal project has been created, remember you are able to filter for internal projects and specific internal clients in the reports section. This means you can easily distinguish between internal project resource consumption vs external resource consumption.