Ghoul

During that firefight in Philly, we see him shot multiple times, and he just brushes it off, much in the same way he brushes off getting shot with the tranquilizer dart that Lucy fires at him.

That's not really how ghouls work.

Ghouls aren't immortal to everything.

They're immortal to invisible forces, to natural causes.

That said, the entire process of ghoulification hasn't been overtly defined and explained to us by the Fallout franchise.

Oh no. They'll kill me if they find out.

But this isn't how being a ghoul works.

Ghouls don't grow back body parts.

Ghouls don't heal themselves.

When they get damaged, they keep their damage, which is why the ghoul had to sew his finger back on.

He didn't regrow a finger.