Alienware is the sticker Dell put on their more expensive systems. Origin is where the Alienware executives moved to when they sold their company to Dell. But I’ve not really heard that much about either.
TBH, I’d work out what you want in the box first and then pick the group who assemble it second based on who does good prices for what you want. Although companies as big as Dell do get their own custom parts (rather than buying an off-the-shelf motherboard from one of the big 4 manufacturers), they are still paying for someone else to actually make the thing you’re buying. So really it’s best to decide what you want and which manufacturing companies you’re going to trust to have good quality control first and then look for who puts a sticker on the box at the end once assembled based on price and warranty offering.