Yes.
All Macs have several USB ports. The iMac, for example, has some on the back and one at each end of the keyboard.
Plug the flash drive into one of the iMac USB ports. An icon will appear on the desktop. Right click the icon and then click open. If it does not appear on the desktop open finder and look for the flash drive on the far left under devices. Click it and it will open.
Older iMacs use a USB keyboard which plugs into a USB socket on the back. Some newer iMacs use a Bluetooth wireless keyboard. These need to be turned on by the switch at the right hand end. A light indicates if the keyboard is working. If the light flashes it means that the keyboard has not yet connected to the computer. (See links below)
The original iMac was released in 1998. It was made by Apple Computers, and was the first computer Apple ever made which used the concept of art. It came with revolutionary USB ports and came in lots of different colours. Apple have been making iMac's ever since, with the latest one having a 27 inch Monitor.
You will have to purchase a USB card reader. They are very inexpensive and can be bought anyplace that sells computer accessories.
You do not make the iMac, Apple does.
The original iMac personal computer was the iMac G3. This model was released in 1998 and discontinued in 2003. One can purchase a used iMac G3 on eBay or Amazon.
The usb cable extension is made by another usb cable. The cable is made with one usb connecter and 1 usb connecter. You can simple attach the extender cable to your usb.
One can see old iMac Commercials in Youtube which features the different types of commercial about old iMac such as All Mac vs. TV adds, Apple iMac vs. HP, and iMac G3 starting up.
If your laptop is equipped with a USB port (most are) then virtually any USB wireless adapter will work.
Millions of people have an iMac. Lots of us have more than one.
No, an iMac G3 is an all-in-one computer.