OP, the W_Healthbar asset updates its own health. There is no code that calls it or updates it.
Look in Content/UI/Hud/W_Healthbar and take a look at its Event Graph.
When the Healthbar constructs, it hooks into the Player Controller’s OnPossessedPawnChanged event, and then immediately calls the OnPossessedPawnChanged event with the current pawn.
In that event, it registers itself to receive OnHealthChanged events from the Pawn’s Health Component.
From then on, any time the pawn’s health changes, the widget automatically updates itself.
When the player dies and respawns, the health widget automatically re-registers itself on the new pawn’s Health Component.