I know that in the case of HP, different brands of internal parts are used for different classes.
The last time I did thorough research for buying a pre-built computer was in 2005 / 2006, so things may have changed since then.
But at that time, the lower end HP computers, ($700 or less), used Toshiba hard drives and optical drives, and either Toshiba or Compaq brand memory.
Compaq memory was mostly used throughout the mid range, ($700 to $1,300), with a mix of the lower or higher end parts.
And the higher end HP computers, ($1,300 and up), would have Hitachi hard drives, Toshiba optical drives, and HP memory.
I don’t know what other brands were used for things like fans, monitors, ports, etc.
But it’s probably similar, expensive parts in the higher end, cheaper parts in the lower end, and a mix in between.
Other brands likely do the very same…
That may even be why Dell’s in the UK seem to be made better, because the company stands to make more money per unit there, thus they use better parts.