Alternators, starters, sensors, wiring and parasitic draws.
Electrical faults are where a lot of shops give up, because tracing one takes patience rather than parts.
A battery that keeps going flat overnight is usually a parasitic draw, something still pulling current after the car is shut off. We find it rather than selling you another battery.
Lighting, power windows, wipers, charging faults and intermittent no starts all land here.