Q1: Why can not Miracast/airplay your phone's screen to the projector when your phone connected to the projector via Bluetooth?
A1: The projector only can be connected to some simple Bluetooth devices such as the mouse/speaker/keyboard and gamepad and so on which support Bluetooth. If you want to Miracast/airplay your phone's screen the projector must connect to your phone via WiFi or HDMI cable and achieve to transmit the large code stream of data.
Thank you for your understanding and support.
Reference information: https://toumeipro.com/pages/The-manual-of-c800-mini-dlp-projector