Who is on the other side

Reading order flow means recognizing the footprints of a few families of participants. None of them is "the market" on its own, but each has constraints, and those constraints are what create the setups.

Market makers

They quote both sides permanently and earn the spread while managing their inventory. When they are too loaded on one side, they shift their quotes to rebalance. When risk rises (macro announcement, volatility), they pull back: the book thins out, the spread widens. Their footprint: walls that move with price, and liquidity that evaporates just before the news.