how can I connect a USB thumb drive to an iMac USB-C?

You need a dongle that will allow a connection between USB-A and USB-C

https://www.apple.com/shop/product/MJ1M2AM/A/usb-c-to-usb-adapter

One option is an adapter from Apple: https://www.apple.com/shop/product/MJ1M2AM/A/usb-c-to-usb-adapter

With an adapter, please get one. You can find many to select from on Amazon. You will need to look for a USB A or B to USB C adapter.