That iMac has two USB-C (USB, DisplayPort, Thunderbolt) ports and four USB-A (USB 3.0) ports. All are capable of USB 3.0 speeds. You could plug a flash drive with a USB-A connector into any of the four USB-A ports, or one with a USB-C connector into either of the USB-C (Thunderbolt) ports.
However, flash drives are a poor choice for long-term storage of data. They're easy to lose and don't have a very good reputation for reliability compared to external hard drives, or to external SSDs. (Both flash drives and SSDs can also theoretically have data "evaporate" if you leave them sitting unpowered for years.)
I would recommend using external hard drives to back up your Mac. Copy that 15-minute video to your Mac, and to your backup drives.
If you want to do so, you can also put it on a flash drive – but maybe go back once a year and check that the flash drive is still working. If it isn't, then you'll have the copies of the video on your backup hard drive(s) with which to prepare a new, working flash drive.