Apple is cracking down on apps that overtly claim to download YouTube video to your iPhone or iPad, but it's still possible in iOS 10 and iOS 11.
We're using Documents 6, which is available only in the UK App Store. We're unaware of an equivalent for US users.
However, an alternative for US users is to download those videos to a PC or laptop, then transfer it to your iPhone or iPad's Videos app.
We also explain explain whether it is legal to download YouTube videos.
Install the Documents 6 app
On your device, go to the App Store and search for "Documents 6". This is a free file management app that'll let you download YouTube videos.
Once installed, launch Documents 6 and tap the bottom-right icon that looks like the Safari icon (a compass).
This will display a web browser where you can search for "savefromnet" (without the quotemarks).
Tap on the en.savefrom.net link in the search results.
Go to YouTube
You need to get the URL - the web address - of the YouTube video you want to download. One of the easiest ways is to launch the YouTube app on your iPhone.
Find the video you want and wait until any advert has finished playing and the video begins. Tap on the video to show the controls, then tap the share icon (an arrow pointing right).
Now tap Copy Link.
Return to the Documents 6 app, and tap in the white box to select it. Tap again to bring up the Paste option and tap that to paste in the URL you copied from the YouTube app.
Download the video you want to watch offline
Tap the green tick and you will see a list of various resolutions, so you can choose the quality you want.
Tap the green Download link next to your preferred quality and rename the video if you want to. Now tap Done and the video will download. It's best to do this via Wi-Fi as videos can quickly use up your monthly data allowance.
Finding more videos you'll like
Once downloaded, you'll see the video in the Downloads section of the Documents 6 app.
You can save this to your iPhone or iPad's Photos app (not the native Videos app, unfortunately). To do this, tap and hold on the video and drag it up until you see the folder list. Keep holding and drag it into Photos and then into Camera Roll.
Now you can just head to the Photos app and watch the video.
Alternative method: Download YouTube video to a PC or Mac
If you don't like the hassle of copying and pasting links and switching between apps then there's an easier way if you have a PC (or a Mac) using Softorino's YouTube Converter.
Head to the YouTube Converter website and download the version you want - Windows or Mac. The downside is that this app isn't free - it costs $19.99.
Connect your phone or tablet to your PC and tap the 'Trust' option if asked.
An alternative is YouTubeByClick which is free with an optional premium mode that gives you more control over the quality of the file you download. This won't transfer videos directly to your iPhone or iPad, so you'll have to do that via iTunes or other software.
Transfer the downloaded video to iOS
Now head to a web browser and copy the URL of a YouTube video you want to download. The video will automatically appear in the YouTube Converter program (and YouTubeByClick also works in the same way). You can copy more URLs and build up a list of videos to transfer to your iPhone or iPad.
You then need to choose the quality for the videos. Although you can select 4K, it's more sensible to choose 720p or 1080p as no iPhone or iPad yet has a 4K screen, and your chosen video may not even be 4K.
Click the Convert and Transfer to iPhone button and the videos will be downloaded and transferred to your iPhone's Videos app. It can take a while, so do this well in advance of needing the videos.