re: Need help replacing a doorbell. Can’t seem to shut the power off.

If it's an old bell, it gets power via a 24V transformer probably bolted to a junction box in the attic. The circuit that converts the 24v AC to DC must have a large capacitor of possibly a battery that keeps the light burning when power is cut.

If you want, throw breakers for attic lights and wait a while. No real electric hazard.