How add custom system work to CV?

I have spent over a decade working on some internal custom systems. As part of that, I've done some really good work I would like to highlight on my CV/resume.

These are systems that very few people know about.

How do I explain them on my CV? When no one knows what I'm talking about?

People care what your system did on a very tiny scale:

• used a SQL or Oracle database
• was written in C++ or Rust
• wrote millions of tiny text files
• spawned agents that ran on other machines

These kinds of details show your technical abilities.

And they care on a really big scale:

• saved the company a million dollars a year
• enabled the same staff to handle triple the previous volume
• reduced error rate from 17% to 3%
• reduced time for reaction to new legislation from 8 months to 2 weeks

These show you understand the business impact of the work you do and why it matters.

They don't know or care on the intermediate scale, which is what you seem to be concerned about

• calculated the tolerances for the flubblegabber
• renormalized the fingalink for new spidspods
• ensured the bobbaloo is always within range except on weekends

If you want to include a sentence that puts your system into context, go ahead, but understand it will read like one of the above to most people. That's fine. Focus on the large and the tiny. Explain your systems from those two lenses.