Water leak during subzero
The water to fill the water heater comes from the cold supply to it and the rest of the house - there's not normally any way to fill it without supplying water to the rest of the house, unless you have valves to shut everything else off, and have shut them all off.
How long your water heater takes to fill depends on the size of the heater and the flow rate of your supply. You need to open a hot tap to vent the trapped air in the heater, to allow it to fill.
You may have other leaks you have not fixed - you will have to look for those, we can't see them from the other side of a computer screen.