External Memory not found by MacBook Air

Don't confuse external storage with memory. Two different animals. ("Memory stick" is an unfortunate but common nomenclature for that type of external storage.)

Make sure you have enable the Show External disks on the desktop in the Finder Settings > General tab.

See the devices connected to your Mac - Apple Support

Do you know if the hub is working properly? Have you checked it with another computer?

Launch Disk Utility and see if it reports seeing those external drives.

If it does see them, select each and run First Aid on it.

Have you used the SDcard and USB stick before with your Mac?

Note that if you try plugging the USB flash drive directly into the Mac (possibly via a USB-C to USB-A adapter), you might need to “approve” its use as a data peripheral.

This is a security “feature” found on Apple-Silicon-based Mac notebooks.

Does the hub have its own power supply? A hub that gets all of its power from a computer host port often will not have enough to go around to meet the demands of multiple devices.