Apple makes it sort of difficult to download files to your iPhone or iPad, so you'll use an app called Readdle. Here's how to get it:  Open the App Store. Tap Search at the bottom-right corner. Type readdle into the search bar at the top of the screen and tap Search. Tap GET or the cloud icon next to "Documents by Readdle." It's the gray "D" icon with yellow and green accents. Follow the on-screen instructions. It's the white icon with a red rectangle and white triangle inside. Tap the magnifying glass at the top of the screen to search, or tap Library at the bottom-right corner to browse videos you've saved. Once you get to the video, it will start playing immediately. It's the icon with an arrow below the video's title. Several sharing icons will appear. It's the gray icon with two overlapping rectangles in the bottom row of icons. This copies the video's link to your clipboard. It's the gray "D" icon with yellow and green accents. It should now occupy the last icon position on your home screen. Since it's your first time using the app, tap Continue when prompted, and then proceed through the screens until you reach the screen that says "Documents" at the top. It's at the bottom-right corner of the "Documents" screen. This opens a web browser. You can do this by typing the URL into the "Go to this address" bar at the top of the screen and then tapping Go. This springs open your keyboard. After a second or so, the "Select All" and "Paste" options will appear. The YouTube video link you copied will appear in the blank. Keepvid will locate the video and provide some download options below. It's the blue button below the video's duration. The "Save File" screen will appear. If you want a smaller file, you can tap the Download Other Formats button below instead and choose something else. This downloads the video to your iPhone or iPad. Once the video is downloaded, you'll be returned to the main Documents by Readdle screen. If you add Readdle to your Files app it'll be easy for you to access your downloaded videos. Here's how:  Open the Files app (the blue folder) on your home screen. It may be buried in a folder. Tap Browse at the bottom. Tap Edit at the top-right corner. Slide the "Documents" switch to the On (green) position. Tap Done at the top-right corner. You can now use the Files app to access the videos you download with Documents by Readdle. When you're ready to watch your video(s), follow these steps:  Open Files. Tap Browse. Tap Documents. Tap Downloads. Tap the video to watch it.

Summary:
Install Documents by Readdle on your iPhone or iPad. Open YouTube on your iPhone or iPad. Go to the video you want to download. Tap Share. Tap Copy link. Open the Documents app. Tap the blue compass icon. Go to https://keepvid.pro in the browser. Tap the "Enter Link" field. Tap and hold the "Enter Link" field. Tap Paste. Tap the blue Download Now button. Scroll down and tap Best Download. Rename the file (optional) and tap Done. Add the Readdle folder to the Files app. Watch the downloaded video.