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Most engineers think of money as a number in a database. In reality, a 'dollar in your bank account' is a claim against a regulated financial institution, mediated by a settlement network, governed by clearing rules, and ultimately backed by a central bank. Building a money product means working WITH all these layers — getting any wrong is a compliance failure.
The actual layers behind a $10 transfer.
User intent: "Send $10 from Alice to Bob"
Layer 1: APPLICATION (your code)
- REST API call: POST /transfers { from, to, amount }
- Idempotency key, audit log entry
Layer 2: ISSUER/PROCESSOR (Stripe, Adyen, Razorpay, etc.)
- Validate, decide routing rails, generate tx ID
- Hold or escrow if needed
Layer 3: SCHEME / NETWORK (Visa, Mastercard, NACHA, SWIFT, UPI, SEPA)
- Authorize, capture, settle the transfer over its rails
- Different rails = different speed/cost/reversibility
Layer 4: BANKS (Alice's bank, Bob's bank, correspondent banks)
- Debit Alice's account, credit Bob's account
- Settle through central bank or interbank netting
Layer 5: CENTRAL BANK (Fed, ECB, RBI, etc.)
- Final settlement at the central-bank reserve level
- This is the ONLY layer where the dollar "truly moves"
Most "real-time" transfers (Venmo, Cash App) move at Layer 1-2 instantly
but only settle at Layer 4-5 hours or days later. That gap is where fraud,
chargebacks, and counterparty risk live.