The Twitter emoji, a.k.a. Twemoji, source is available at https://github.com/twitter/twemoji.
Google emojis are bundled with the Noto Color Emoji font. The source images can be found at https://github.com/googlefonts/noto-emoji.
Noto Color Emoji is part of the Google Noto Fonts family which comprises over 100 individual fonts. The Noto fonts support almost all Unicode scripts.
The Apple emoji are included in the Apple Color Emoji font. The font is only distributed with bitmap emoji glyphs, and no vector sources are available online.
Interestingly, the Apple Color Emoji font contains a number of Easter eggs one of which is the text seen in the open book emoji.