The source code for the TM SGNL apps (basically a backdoored version of Signal used by Trump officials) is public! Since it's open source, I've pushed it to github for easier research https://micahflee.com/heres-the-source-code-for-the-unofficial-signal-app-used-by-trump-officials/
iOS code: https://github.com/micahflee/TM-SGNL-iOS
Android code: https://github.com/micahflee/TM-SGNL-Android