Why are some emoji not displayed? I am wondering, on what does it depend which emoji are available on which system? I was using an emoji from the Supplemental Symbols and Pictographs code page and it displays on OSX, but not on Android. Looking at the chart from the Wiki article, I can see that it displays in full on OSX, but only some of the emoji are visible on Android Nougat.

Why is that?
It depends on the NotoColorEmoji.ttf font file you have installed. If that file doesn't include a glyph for the Unicode code point you want, it can't show it. Here's an SO post about fonts and glyphs:
https://stackoverflow.com/questions/3582944/how-does-a-unicode-character-get-mapped-to-a-glyph-in-a-font

This reddit post talks about replacing your system's NotoColorEmoji.ttf font file with one that includes EmojiOne emojis:
https://www.reddit.com/r/Android/comments/3xezb9/emojione_on_android/