> But that’s precisely what it allows you to do

And if I don't want a building block like logind or journald, can I not use them? Can I take those blocks elsewhere, like over to FreeBSD and use them there?

SystemD is not "building blocks" — which implies modularity — it is a monolith: components cannot be added or taken away or replaced. The best you can do is maybe run disable --now. (I'm at least thankful I can usually do an apt purge resolved.)