Use YouTube Premium to download videos in the YouTube app for offline viewing
Open the YouTube app
Sign in to your account
Open the video you want to save
Tap the Download button below the video
Choose the video quality if prompted
Wait for the download to finish
Open the Library or Downloads section to watch it offline
Keep in mind that downloaded videos are available only inside the YouTube app
Keep in mind that some videos cannot be downloaded
Keep in mind that offline downloads may expire and need periodic reconnection to the internet