I do not live in a condo thus I have no need to shut the main valve
Backflow from the sewer or drain pipes.
Which water you talking about? Under the kitchen sink there should be a C/W shut off If your talking toilet there should be a shut off left hand side under the tank. Shower or bath tub possibly integral stops Main cold water shut off Normally by the house trap. Multi story building a cold water riser
If you have your own water heater there should be a shut off at the heater. If not I would look under the kitchen sink. There may be shut off valves at each fixture. If there is a central water heater in the basement, there would be a shut off to each unit.
With the water main shut of valve of course
Showers and tubs do not usually have shut off valves on the supply lines. To shut the water off you will have to shut off the main water line. There should be one on the cold water line to the water heater. This may shut both the hot and cold off or just the hot. There will be one in the main line coming into the house on the basement wall closest to the street or at the water meter.
You will get some back pressure from the water heater. It will come out of either the hot or cold. Shut the tank off if there is a shut off there.
Most tubs do not have a shut off in the supply lines to it. You have to shut the main off where it comes into the house.
The water meter is shut off? The main water line is shut off? The valve for that faucet is turned off? The waterline is made of galvanized piping and has corroded shut? Aerator is clogged, remove and clean or replace.
Outside at the main shut off valve. Showers/Bathtubs do not normally have a shutoff valve like sinks and toilets. The main water valve for the house must be turned off.
Tubs usually do not have shut off valve for them. You have to shut off the main supply. Depending on how the house is plumbed, you may be able to shut the cold supply to the water heater and that will shut off both the hot and cold or the main line where it comes into the house.
The valve seat or washer on the main valve are not seating/sealing properly.
No. The Main is source of water for that as well.