[Mr_P]

This is literally the 'O' in SOLID.

The key idea is to break code into "chunks" that each do one thing.

Then, if you have to add a new feature, it goes into another chunk, instead of editing/modifying existing code.

The same logic applies to system design at different scales, whether fine-scale OOP or coarser-scale (micro)service architecture. The ideal size of an individual "chunk" is somewhat subjective & debatable, of course.

It's like Haskell-style immutable data structures, but applied to writing the code, itself.