We are quickly entering the era of 4K as the standard resolution, and there is new 4K content available on YouTube every day. Unfortunately, you cannot watch these videos offline in full resolution using YouTube's standard options.
How to Download YouTube Videos – Complete Guide
YouTube Premium members ($12/month) can save videos and music for offline playback on their phones, but only at a maximum resolution of 1080p. YouTube creators can download MP4 versions of their uploaded videos, but only at a maximum resolution of 720p.
However, that doesn’t mean it’s impossible to download 4K YouTube videos to watch offline. There just so happen to be several third-party apps that let you download YouTube videos in full, in the highest resolution possible, including 4K and 8K.
The most reliable app I have come across is 4K Video Downloader for desktop.