How do brokerage firms provide liquidity?

External question — Quantitative Finance Stack Exchange Author: Kmd Original post: https://quant.stackexchange.com/questions/69937 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. Do they directly put limit orders on the order book? Or do they automatically fill limit/market orders from customers and have offsetting position with their market maker(LP) to offset their net position?