One option is to use an internet browser. First select your flash drive as the download location. How you change the download location varies depending on the Internet browser you use. If you don't know, do an internet search for how. Then log into www.icloud.com, click on the Photos app there, then select the photos you want to download. You can use shift - click to select a range of photos (the last I heard, the maximum was 1,000 photos at a time). After you have selected phones, click the download icon near the top right of the screen (seen screenshot below)

There is also an option to download photos using iCloud for Windows but I believe it will only allow you to download them to your hard drive and you would then have to copy them to your flash drive. See the information below from the PC section of "Download copies of your photos and videos from your device" in Archive or make copies of the information you store in iCloud - Apple Support
On your PC, make sure that you set up iCloud for Windows and turn on iCloud Photos. Open File Explorer. In the Navigation pane, click iCloud Photos, then select the images you want to keep on your PC. Right-click the selection and choose "Always keep on this device." If you use iCloud for Windows 10 or earlier, learn how to download your photos. After the items download, copy them to another folder on your computer. To do this, press and hold the Ctrl key and drag the items to the folder. Include this folder if you back up your computer.