When you model a business feature in C#, how do you decide what should become an object and what responsibility it should own?

In production I test whether the model reduces invalid states and change blast radius. If one rule change touches many unrelated objects, the responsibility is fragmented; if one object knows persistence, email, pricing, and authorization, it is overloaded. Aggregate boundaries follow consistency and concurrency needs rather than the shape of an object graph. I evolve the model from real use cases and repeated changes, keeping public behavior stable while internals improve. The goal is understandable ownership, not a perfect mirror of business vocabulary.