• Foundational (fundamental or underlying)
  • Machine-level (or, informally, "bare-metal programming ... is considered to be a low-level method of programming that is specific to the hardware used in the computer system and is often used for optimizing software and applications for an individual computer")
  • System internals
  • System-level
  • Underpinnings (support or foundation)

A few I can think of:

  • "Fundamental"
  • "Core"
  • "Kernel"

I think foundational and core are quite good options. I'd just add to the list the word infrastructural. It may have more of the "technical tinge" you're looking for.