Apple Music M4A high quality songs

How do I know which songs are mastered in the higher sound quality AAC/MP4 sound format. Are these better sounding versions available for standard Itunes MP3 purchases or is the format limited to Apple Music subscribers/songs? in what way do these songs mastered in this format excel compared to MP3? What are the compared file sizes for each format roughly or proportionally to each other. For songs that I have already purchased, is there a way to download the MP4 version while in Itunes because I don’t think Ive ever seen anything that says MP4 on it. I’ve seen the ‘’ Mastered for Itunes logo on certain songs. I basically want to download all of my library in the higher quality MP4 format vs the MP3 format they’re all coded in now. Thanks for the repy.