ETF market-making has a more complex mechanism than 'regular' market-making. This answer deals explicitly with open-ended ETFs. I will refrain repeating the details about market-making that most people know that have already been asked and answered ad nauseum.
On top of traditional market-making in the secondary market in which traders trade with each other, open-ended ETFs involve what is known as an authorized participant (an AP). An AP can be a market-maker, a third party that market-makers work with or they can be large broker-dealers that act in both capacities. An AP will create and/or redeem units of the ETF's by working with the ETF sponsor in a predefined role. The create/redeem mechanism is the one by which open-ended ETFs outstanding shares change each day. This is also the mechanism that keeps the open-ended ETF within a general boundary relative to its NAV. All open-ended ETFs have multiple symbols that can be watched in real-time that disseminate all the information about them. For example, SPY is just the symbol that traders use to exchange shares of the ETF. There are also several other symbols that can be monitored if one were curious. SPY's NAV can be watched with ticker SPY.NV, SPY's intraday value can be watched with ticker SPY.IV. There are also symbols for SPY's shares outstanding and total cash per creation unit. These symbols will vary depending on the data provider. The SPY.NV, SPY.IV relationship to the general symbol SPY will tell you how active the AP's are and how much supply or demand there is outside of the secondary markets.
If an ETF is trading at a premium or discount to it NAV, it represents an arbitrage opportunity for someone to buy or sell the underlying components, tender them to the AP in order to create or redeem a unit, and then buy or sell said unit to lock in the arbitrage. Individual investors and hedge funds can establish relationships with prime brokers that can act in both capacities enabling them to buy, sell, and sell short large amounts of ETF shares without moving a market much at all.
BUY example: If I am a hedge fund that wants to buy 1 million shares of an ETF, I could ask my broker to create the units for me. They would buy shares of the underlying components, submit those shares to the sponsor, and dump the newly created ETF shares into my account. Depending on the nature of the relationship one has with a broker, they may do this for a flat fee or some other type of fee structure.
SHORT example: The creation mechanism also allows large traders or hedge funds to sell short ETF shares that didn't previously exist. The AP will create a unit of the ETF (say 50k shares to a unit), delta hedge it internally to offset the firms exposure and loan the new ETF shares to a client. They make money by charging a borrowing fee for the service that exceeds their cost of creating the ETF unit and maintaining the delta neutral position. The borrowing fee can change daily to allow the AP to maintain their desired profit margin.
There is a bit more to understanding how AP's play a role in making markets with open-ended ETFs. I have added some references below for you to read through. What I have described here is a somewhat high level explanation.
References:
Open-Ended ETFs vs. Closed-End Funds vs. Open-ended Mutual Funds
Understanding Exchange-Traded Funds: How ETFs Work
Authorized Participant
A Primer on ETF Primary Trading and the Role of Authorized Participants
