Where do mini and micro lots come from?

Do brokers group their funds and combine mini and micro lots into a standard lot? If someone bets a standard lot and someone else asks for a mini lot, then how are the units distributed?

If you’re trading through and ECN then the trade is matched up size for size with someone else in the network. If you’re trading through a dealer then the broker is on the other side and aggregates all longs and shorts into a net exposure which often is then hedged away.