Click buy and your order does not sail off into some neutral arena called the market. It lands on your broker's server, where software decides, in microseconds, what to do with it: pass it to a bank, net it against another client's order, or simply hold the other side. Most traders never learn that this decision exists. Learning it explains almost everything that otherwise seems mysterious about retail trading: why spreads breathe, why fills slip at 8:30, and why the same order costs different amounts at different hours of the day.
How the house gets paid
Broker revenue comes from three visible streams. First, spread markup: the broker receives a raw spread from its liquidity sources, perhaps 0.2 pips on EURUSD, and quotes you 0.8, keeping the difference on every trade. Second, commissions: raw-spread accounts typically charge around $6 to $7 per standard lot round trip instead of marking up. Third, swap: a financing charge on positions held past the daily rollover at 5pm New York, which disciplined day traders rarely pay because they end the day flat. Notice the shape of this business: the broker earns per transaction, win or lose. A client who trades 40 lots a month for years is worth far more than one who deposits $10,000 and burns it in six weeks. Whatever you may suspect, the arithmetic of a legitimate brokerage prefers you alive.
A-book, B-book and the hybrid reality
What happens to the order itself follows one of two models. A-book: the broker passes your trade through to liquidity providers and earns only the markup or commission, carrying no market risk itself. B-book: the broker internalizes the trade, taking the other side onto its own book, so your loss is its gain and vice versa. B-book sounds sinister and is more mundane than it sounds: because a majority of retail flow loses, internalizing small orders is profitable on average, and it also allows instant fills without touching the external market. Nearly every large retail broker runs a hybrid: small and short-lived flow is internalized, while large, consistent or sophisticated flow gets hedged or passed straight through to liquidity providers. Your trading style, in effect, decides which book you live on.
What the liquidity provider sees
Behind the broker sit liquidity providers, banks and specialist market-making firms, streaming continuous two-way prices. Their business is earning the spread thousands of times a day while avoiding being run over by better-informed flow. That defensive instinct is something you experience directly: in the seconds around a major data release, providers widen their quotes or pull them entirely, which is why every retail platform on earth shows EURUSD spreads jumping from under a pip to 3, 5 or more pips at 8:30 New York. Nobody is targeting you personally. The wholesale price of immediacy has spiked, and the retail quote is passing the bill along.
Trading so the plumbing works for you
You cannot choose the broker's routing, but your behavior decides the quality of your fills. Trade liquid instruments in liquid hours: a EURUSD market order at 10:00am New York fills at the quote almost every time, while the same order five seconds after a jobs report can slip several pips. Keep size sensible relative to the market: one lot passes unnoticed, while oddly large orders in thin hours get worse prices for honest mechanical reasons. Use limit orders where the setup allows, taking the spread rather than paying it. And stop trying to scalp the first seconds of scheduled news, which is the one activity where every layer of this plumbing, spreads, slippage and pulled quotes, is aligned against you at once.