Where does Ekkos damage come from?

I feel like everyone and their mother is playing Ekko reroll, so I've been looking into the unit. On paper, it doesn't look like Ekko should do as much as it does. It's BIS (Titans, BT, & rageblade) don't seem like they should be contributing to its damage in the way it currently does. What is letting Ekko carry as hard as it has been? Is it Zaun, Disruptor, or am I just wildly misinterpretting his ability and it scales better then it seems?