Who makes HP/Compaq laptops?
Although the major players tend to keep this type of info on the DL for obvious reasons, I can say with some certainty that Arima was a major ODM for Compaq's higher end units. So much so that they had a facility in Houston that did nothing but service those machines for Compaq. However, pretty quickly after the HP take-over, Arima was phased out for other, less expensive contractors and that facility started scaling back as more and more of their models went out of warranty.
Quanta has been one of the big ODM manufacturers for Dell and still is, although they even did some work for IBM before the Lenovo buy out. Its not as easy to tell for the "big boys" since they have enough clout to make certain that no ODM labels are apparent on the machine, unlike boutique manufacturers such as Alienware (prior to their Dell acquisition), when their machines would have a Clevo label (complete with model number) right on the bottom of the unit! That being said, if you are attentive and look a few machines from various companies, you'll find that certain ODMs will use the same design/manufacturing processes such as having the keyboard attachment screws underneath the power panel just about it.
But in a general sense, most of the major "OEMs" will have multiple "lines" intended to sell to differentiated markets and those lines are often contracted out to different ODMs. Even back when IBM had their machines built in their own facility in Mexico, their "iSeries" machines were contracted to Acer in an effort to get into the "consumer" market with a less expensive machine.