Four USB 3 ports (compatible with USB 2) 27" 2019 19.1

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.

My opinion on them all is none are appropriate for any kind of storage, transfers maybe, but not storage. They're simply too unreliable compared to an actual hard drive, small and easily lost and difficult to label.

As is the usual case, YMMV and we all have our favorites and opinions.

I'm 77 btw and some tech stuff is over my head too.

I have to heartily agree with ku4xh, do not rely on Flash drives to store anything mportant, all files on them can disappear in a flash!:(

https://eshop.macsales.com/shop/external-storage#interface-ports:usb-3-0-usb-3-1-gen-1