That looks interesting but? 1) If you use the option to download it to your browser, the 4K quality has no audio? 2) If you choose to use their app, you have to pay?
2Conv can be a good option as well. It is a free online tool to save YouTube videos in MP3, MP4 and AVI format, supporting URL. If a certain video cannot be converted by 2Conv, I would use Joyoshare Screen Recorder I installed before as an alternative to record videos in 1080p without restrictions.