AFAIK, and this is off the top of my head so you might want to fact check the fine details:
Hitachi hasn’t produced exclusive components for their drives for a couple years now.
Their HDD division is mostly Western Digital with some assets divested to Toshiba (Hitachi owns a good part of WD in exchange). All their 2.5 and SSD are WD, and their 3.5" are largely Toshiba… which happens to have WD make them for them anyway (while Toshiba still does its own 2.5 and has for quite a while, as they entered that market from the laptop end of things).
Pretty F-ed up actually.
Their control electronics used to be re-branded Matsushita (don’t know if it’s still the case as that was due for dismissal or renewal Q4 2013 I believe), and their manufacturing has moved to WD.
WD and Seagate have almost a duopoly on magnetosensitive parts, with Toshiba having entered actual magnetic printing only a year or two ago and not covering their entire stock.
Electronics are a bit wider, but still largely coming from one of three manufacturers
Basically the only reason you might want to buy Hitachi is to pay a smidge less for a Western Digital but have a much less safe warranty and a higher chance or non-replacement, whereas WD is known to be very customer friendly with straight replacements.
There isn’t a real point to buying Hitachi over WD or Toshiba, really, any of their drives will be using either set of parts.