For Downloading Video From You Tube or From any Other Site, "Tube Mate" is the best app and it is free you can Download The Full HD Video with This App. It is Avelabel for Andriod Users. But It is Not Available On The Play Store.
I would suggest you use any of the good online downloaders. They can pretty do the job than most apps with limited access.
Newpipe can be a video downloader as well as a youtube app alternative, used it quite a long time before starting to use vanced youtube.
SnapTube is the one for me. Been downloading videos in .mp4 format. Not sure if the video is H264/AAC encoded.
Actually VP9 and Vorbis are better than H264 and AAC. VP9, Vorbis can have better quality in same bitrates compared to H264,AAC. and WEBM and MKV are just containers and they both have same codecs inside in youtube videos. If you are downloading videos for offline purpose then you should stick with vp9 and vorbis (webm, mkv)