“whether the ferries are primarily civilian ships converted to military use, or military ships used currently as civilian ships? “
They’re civilian vessels that are commandeered to use for these type of exercises. China’s military doesn’t have any real troop transport capacity of its own to speak of, since they have no plans for any long range invasions they can’t conduct over land. Taiwan, and perhaps Japan, are the only real targets they might invade that would require naval transport, and I think they can’t justify spending the money for that capability when they can just commandeer civilian vessels instead.
I have wondered if the specs when they are built are much more robust than we would have for our ferries? I am thinking of things like hard points and compartmentation.