You can see various threads where people complained about the forced Apple emoji e.g. https://github.com/signalapp/Signal-Android/issues/3712 and https://github.com/signalapp/Signal-Android/issues/3675
Signal's own FAQ says
> Signal Android includes built-in emoji functionality for consistency between platforms
https://support.signal.org/hc/en-us/articles/360017561992-Wh...