Adding liquidity means to place an order that sits on the book while removing liquidity means to execute against an order that's already resting on the book.

> What's the mechanism for removing liquidity?

Liquidity removal = market order

Liquidity providing = limit order (not immediately executable)

The order type is a red herring (you can take liquidity with a limit order).

The only difference between an order that removes liquidity and an order that adds liquidity is whether it executes upon arrival (removing liquidity) or rests on the order book on arrival (adding liquidity).