You and I must work in very different contexts, as these questions are so obvious that they first seemed like satire to me.

You enforce API contracts in a monolith (or any codebase, really) via an at-least-modest amount of typing and a compiler. You diagnose performance issues via any number of tools, prominently including metrics and profilers.

My context for this is a lot of years working with backend languages like Java, Rust, etc. though the same assurances and tooling are available for most every platform I’m aware of.