You’d own money movement. Ledgered internal state, transaction lifecycle management, idempotency, retries, deterministic replay. Continuous reconciliation across independent ledgers, card network events, banking events and third party reports, and the tooling to chase a mismatch to its root before it compounds.
You’d build the guardrails that stop bad states happening rather than catch them late: double spend, duplicate webhook processing, inconsistent balances, orphaned settlements.
You’d own the integrations with card issuing, banking and disbursement, and credit partners, and the compliance and reporting infrastructure that the banking partner and the regulators depend on.
The system serves hundreds of accounts today. The job is taking it to hundreds of thousands, through spiky demand, without dropping or duplicating a single event. Correctness here isn’t a quality bar. It’s the product.