Architecture isn't diagrams for their own sake — it's the set of decisions that make a system easy or painful to change six months from now.
One source of truth
For every important fact, there should be exactly one place that owns it. In Flayona, the database owns financial state; the UI reflects it and never invents it.
Boundaries you can point to
Keep the layers explicit: presentation, application logic, domain, data. When a change touches only one layer, you've drawn the boundaries well.
Make the implicit explicit
Lifecycles, permissions and workflows should be written down in code — state machines and capability tables — not implied by a tangle of conditionals.
A deeper piece on modular monoliths vs services is in the works.