I am gutting my kitchen in preparation for a remodel project being performed by a contractor. I need to know how to safely disconnect some hardwired electrical connections

I do not mean this wrongly, but if you don't know how to do it, to a point you are asking online, you don't need to do it

Have to agree with Fr_Chuck, Meters can verify hots but need Known good ground or neutral.
Tic Tracers are good for identifying hots with no ground for reference.
If for some reason you had to do it. Turn off power to whole house.
Do your destructive deeds, cap all remaining wires and turn power back on.
You now run the risk of disturbing other circuits.
Best to have person doing rewiring, to do unwiring, have them explain.
Never hurts to learn. It is safer, can actually save you money and your life.
Good Luck