Debian Buster + Jailkit concern

Pretty much. If you add an "app" it copies only the one executable, with dependant libraries; for git you add it to the "sections" and it will with because there is a "git" section in the default jk_init.ini. And yes, it will update all your jails with the global values (unless you overwritten then with site specific values), though that happens a few at a time to avoid huge load spikes, so it might take a bit until they are all done.