The term market maker is used in two different ways.

Some brokers: Sometimes people (especially in the retail environment) use it to describe a dealing desk broker. A broker who doesn’t act as an intermediary but instead tries to match the order internally, sometimes by taking the other side of your trade. Who are those? Many retail brokers. That description, however, is often misleading. The business reality is more complicated than many people realise.

Liquidity provider: Market making generally refers to something else, though. Liquidity provision. Market participants who quote a two-way price (whether you want to get USD by paying EUR or EUR by paying with USD), the difference of that operation is their profit - just like an exchange dealer in a tourist area only electronic (and much more competitive).

Who are those? Many banks, some funds, plenty of HFT shops but essentially everyone posting two-way quotations.