Do you want to download videos from any sources online on iOS without Jailbreaking?
Give Aloha Browser a try. It’s a web browser, but it contains a very powerful video downloader tool that can help you to get any clips online.
First, look for Aloha Browser from the AppStore: (https://itunes.apple.com/us/app/aloha-browser-private-fast/id1105317682?mt=8)

Then search for any videos. When you click play on the video, simply click fullscreen when it is playing. At the bottom right corner, there is a small down arrow button, which is the download button. Select where you want to save the file. All downloaded clips can be exported to Videos app, or any other video players
If you want to secure the app from other people, there is a nice passcode lock feature. It works with Touch ID or Face ID as well (if you have it on your device)
Thanks for reading! Enjoy!