I know Telegram isn't something truly "open" either (and just to mention, neither is Signal) but I don't need to reverse engineer Telegram app. I actually had its repo cloned and synced and had built and tested it couple of times. Though at the time I ended up not using the Telegram as among my contacts few were using it.

My point is open sourced code is much more readable and hence more accessible and you can build it yourself and use it most of the times.