Well if you are looking to amp everything or at least eventually, then determine your budget. There are cheap loc's and expensive ones. If you plan on keeping the oem deck forever and want it to sound the best it can, invest in an loc something like I mentioned like audio control. If it's temporary until a deck, get a 4 channel cheapo loc and use a set of rca splitters to get "6" outputs of audio. 2 would be the front and 4 would be the rear (2 for rears and 2 for sub). All an loc does is take the high level output from the deck and turn it into a low level output to go into the amplifier. This is what the rca connections already are on an aftermarket deck. Except most aftermarket decks that have 6 channel rca outputs have individual control of each set as far as crossovers and sub level control. You can pretty much do this all for an oem deck too with all the right equipment.
I didn't watch the video but, if they ran a 9 wire cable, it was most likely 8 speaker wires which make up 4 channels of + & - for the amp of the mids/highs and the remote turn on for the amplifier's. The remote is what turns power on and off on the amplifier when you turn the deck or car off. So the speaker wires all get cut in half. The wires deck side go into the loc's, get converted to low level and go to the amp input. The outputs of the amp connect to the 8 wires, go back up to the front and connect to the wires that go out to the speakers. The better the loc, the better the signal conditioning. Cheap loc's work but, they are basic basic basic to get the job done. High end loc's have circuitry to condition the signal and give you the best possible outcome of your source.
I'm sure I may have missed some things that others can fill in. I've been out of car audio for years now, so maybe a little rusty and not up to speed.