Aftermarket Amp Help

I am putting in a Dasaita G12 hu in my 2019 Tundra and am trying to install a 4ch amp. I was told to run 9wire from the amp output to the radio harness adapter that connects to the vehicle(After installing and bypass harness on the factory amp). How would I find the wires that need to connect the 9 wire to, and would I have to splice into them in order to do that? I'm 17, and my knowledge only extends to basic sound on less computerized vehicles, lol.
Second question could not just I take the input signal wire from the factory and splice that into the high-level input and then take the factory output wire and put it into the output of the new amp? Thanks!