Open the YouTube app or go to YouTube.com
Find the video you want to download
Check if the video has an official download option (Download button or “Download” in the menu)
If available, tap/click Download and choose the quality/storage option
If not available, use YouTube’s official offline feature (if offered for that video/region/account)
If offered, open the video’s menu and select Save offline / Download for offline
To watch offline, open the YouTube app and go to Library or Downloads and select the downloaded video