Thanks for the good video!
Another approach without using Git SubModules or SVN Externals is described in this Xojo Guest Blog Post: Xojo Projects in a MonoRepo.
That Blog shows a way how to use Text Projects and have Modules (with/without classes in them) to be shared between several Xojo Text Projects.
Sure… a MonoRepo is not ‘the’ solution for every collection of projects - but maybe something to consider.
See also this Forum Thread: Best way to re-use code