The actual app does indeed work with the standard.
RTH is set via the DJI Go app, as are other settings like video resolution/fps. Basically the bird knows its settings. Can't speak for sure for if it is stored on internal storage or not, but it seems to know the RTH from last time. I usually do launch DJI Go first - just to make sure I have settings the way I wan them.
Litchi is pretty much used for setting waypoints, getting them on the device, and sending it on it's merry way.
On my iPhone, Litchi is taking up 77MB.
Also, something I found - you have to have the remote in P-mode to upload the waypoints. It'll bomb if you don't and tell you to do so. So you flip the switch and hit upload again and it'll tell you there is an error and you need to reload litchi. Takeway is to put it in P-Mode after takeoff.
One other annoyance is I can't find any way to cancel the RTH from inside Litchi. It would be nice as it's coming in for a landing to take control or send it on another mission. I supposed I could have the mission end with Hover instead, but cancel RTH would be nice.