Chosen Solution

When I jump my car off battery light comes on then cuts off after pressing the gas but as soon as I take the positive off the hot post the battery needle falls fast nothing on but the car itself all grounds are connected right and hot wires are connected replaced all fuses and relays

Can someone please help me asap I’ve done everything and I can’t figure out what is going on new fuses new alternator new wires from battery and alternator and all grounds are connected and still jump the car off and still dies while running no lights on at all and the battery needle falls fast