I have the following tf tree, but I'm missing the /map frame. Where does it come from? What is supposed to publish it?
and welcome to Robotics Stack Exchange. Could you provide why you would like to create a /map frame? This could be done easily creating a tf publisher using tf2 package like this:
rosrun tf2_ros static_transform_publisher x y z yaw pitch roll frame_id child_frame_id
However, this is just for statics transforms as you can read. Normally the map frame is used to do some localization algorithms, and it wouldn't make sense to say that map and base_footprint transformation is static. Normally, you would like an odom frame as well.