Is there a connection between flag carrier airlines and aircraft manufacturing? That never occurred to me, since most flag carriers fly Boeing and Airbus like everybody else.

But I guess for larger states, like Aeroflot / Air China, they probably subsidize the local manufacturers?

In any case, I'm inclined to push back against your assertion that this is the "same reasons [sic] all other countires" have them - I don't think very many flag carriers are supported because of manufacturing concerns, or that the relationship is similar to the US State's relationship to Boeing. That seems like a stretch to me.