I think the topic was already raised.
The classical way of porting Ubuntu Touch is to use the kernel of Android and using HAL via Halium project we can "easily" mate the interfaces.

Now for Apple's products it would be another story.
One would need to find how to talk with the firmwares. It is called reverse engineering.
Basically you try to snoop on what iOS send to the various firmwares and then try to understand what it should be so you can mimic it for Ubuntu Touch.
Then the porter has to develop a software brick to make Ubuntu Touch talk to the firmwares.
This is tricky and very hard to maintain but doable if one is motivated enough