To your second question, I use a Western Digital 4TB "My Passport" SSD flash drive to store my videos. So far no problems after 2 years usage. Drives do fail, however, and hence the need to have a second backup external drive. You want it formatted for Apple. You can buy it Apple formatted or format it yourself Mac OS Extended (Journaled) using the Disk Utility app found in the Utilities folder on your Mac. Here's a link to the Western Digital website:
https://shop.westerndigital.com/products/portable-drives/wd-my-passport-usb-3-0-ssd#WDBKVX0020PSL-WESN
As for your iCloud question, I don't use iCloud to store or sync my videos -- so can't speak from personal experience. I believe that you can access your videos from the Photos item in the iMovie project sidebar, if you don't have Optimized Storage enabled in your iCloud settings. If you do have Optimized Storage enabled you would need to download the video to your Photos app and then use the Photos item to import it into iMovie. Try it with a test video to see.
Also, you can sign into www.iCloud.com to access your Photos app videoswww.iCloud.com
-- Rich