how to eject flash drive connected to USB to USB-C slot of MacBook Air

Hi, welcome to the Apple Users Community.

Since you mentioned a USB-C slot I'm going to presume you are running Catalina.

1. Go to the "About This Mac" under the Apple icon top left of your screen.
2. Eject any unused USB devices possible, or take specific note of the name of these devices.
3. Which should show up with the "Overview" splash window and 5 other tabs.
4. In the Overview there should be 2 buttons "System Report" and "Software Update" hit system report.
5. This will show a window with 3 drop down menu tabs, Hardware, Network & Software
6. Under Hardware click on Thunderbolt and it should show if anything is connected to those ports. If not move on to 6.
7. Under Hardware click on USB and it will show if anything is connected to those ports.
8. If you do not see the device you is connected to the port under these sub lists then it should be safe to disconnect.

Best of luck

Make sure you have all of these checkboxes checked in Finder's General preference pane:

Then you should be able to see the drive to eject normally.