On the server-side, you need to install pyfcm using pip.
pip install pyfcm
Here is the official documentation for more customization: https://pypi.org/project/pyfcm/
You can send a notification to a single device as well as to multiple devices at the same time just you must have the token or registration id from Android or iOS device. You can manage title, body, color, icon, and many more things of the notification.