I normally use one of the following legitimate methods:
Save it on Facebook: Tap the three dots on the reel to save. It remains in your saved list, but not in your phone's gallery.
Go to "Download your information" : To access your uploaded media, go to Settings → Your Facebook information → Download profile details.
Facebook does not officially allow you to download other people's reels directly, so avoid third-party services, which are unsafe and can steal your info.