Guide Request: Architecture and code design

[bvhme]

From contrib-guide:

• [ ] Architecture

Architecture / Design

• Single Responsibility Principle — a class/function should have one and only one responsibility.
• Three strikes rule — if a small piece of code is copied once and it's simpler, that's okay. Anything more and it should be factored out.
• Code left in a better state than found — we should continually be reducing technical debt, so if you touch a piece of code, it should be nicer than before.