Regarding the word "projection", the thing is that you sync in multiple "sub"-repositories to the metarepo, but you can then recombine them dynamically from this repository. It's a projection as in it's a view, a slice of the metarepo that takes selected bits and combines them into an ad-hoc repository.

The metarepo contains a description of the relationship between those components (who contains whom), such that when you push to one of the contained, depended-on repositories, all the views that include it and as such depend on it get rebuilt.