Tags are just tags. They're created by using them, and followed by following them. Project membership doesn't come into it.
It's up to the person making the post to attach a tag that will find the followers. Eg if you have a post about the House of Westphalia-Pumpernickel it's probably best just to tag it euroaristo.
Tagging with a sub-project makes most sense if it's a self-contained enterprise with a separate crew who might not be following the parent project.
To follow a space page you have to follow one of the tags on the page. Eg for the WW2 pages follow world_war_ii .
But many space pages don't have tags and can't be followed.
There's no way of following other types of pages linked to projects. You can't follow categories.