How do I connect a flash drive to my MacBook Air?

You don't say what year/model MacBook Air you have, but I presume you mean it only has two USB-C ports.

If so, you have several choices. One is a simple USB-A to USB-C adapter. I have these and use them to plug my flash drives (and any other USB-A devices) into the front ports of my M4 Pro mini.

Or, you can multiply your port choices with a hub.

There are also USB flash drives with USB-C plugs – including some that have both USB-C and USB-A plugs. That lets you use whichever plug is most convenient at the moment – so you can, e.g., plug the drive into a USB-C port on your Mac, and into a USB-A port on a self-service photo printing kiosk at a store.

"How do I connect a flash drive to my MacBook Air?: I have a flash drive that I want to connect to my MacBook Air. Do I need to get an adapter ? If so what kind of adapter should I use? I seem to have three ports only."

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Avoid an Adapter All Together:

You can avoid use of an adapter by getting a USB Flash Drive that has both USB-A and USB-C ports on it. They're still catching on, but you'll likely find one on Amazon(usb a usb c usb flash drive), or at your local retailer.