Open YouTube and sign in to your account
Click your profile icon and go to YouTube Studio (for your own videos)
Select Content from the left menu
Find the video you want to download
Hover over the video and click the three dots (More options)
Click Download
Choose the desired format/quality if prompted
Save the file to your device
For videos you own but can’t see Download: check that the video is not restricted by permissions and is in a downloadable state in YouTube Studio
For offline viewing (not downloading a file): open the YouTube app, tap the video, and use Download if available in your region and for that video