Remote Processor Messaging (rpmsg) Framework is used to talk between the operating systems.
Please have a look at the this article.
The Colibri iMX7’s one and only display interface is parallel RGB, Thus HDMI can not be directly connected.
Of course one can design a Carrier Board which converts parallel RGB to the needed display interface. E.g. the Colibri Evaluation Board and the Iris Carrier Board both feature a conversion to analog RGB and to LVDS. None of our carrier boards implement a convertersion to HDMI.
This page gives you access to the datasheet and features of the Colibri iMX7 family.