For general technical and commercial background of the area I would recommend Library Genesis with a wide range of related search terms, including to be exhaustive those in other major languages. You can seed the search query set from relevant Wikipedia topics, book publishing information page listed keywords, dewey decimal categories, or other mature collaborative semantic databases. Using these established semantic databases helps to ensure you don't miss category specific jargon thus undermining your results.
However, arguably the most important element of hardware products from a research perspective is the marketplace state with respect to competition and demand patterns. For this area, I recommend: (1) https://patents.google.com/ (relevant results of which can also be used to seed/re-seed keywords for the above). The amount of information people put in patents is ridiculous, you can shortcut a lot of wasted effort by learning from their mistakes. (2) Existing consumer product databases such as eBay, Amazon, or Etsy. (3) Searching Taobao in Chinese for both the product category (machine translate adequate) and the major functional components. This reveals a true state of costs and supply chain outside of any tightly held specialist manufacturing lines, which is often deeply informative with respect to product design decisions and explaining marketplace state. (4) Researching specific competitors: funding, timeline, team, location, announced marketing and business strategy. Job ads will also give you huge insight in to their skill gaps and technical stack / strategy. (5) Using google trends to identify search volume changes over time within your target market(s).
Hope that helps.