starting guide to make MR

Hi (noob alert)

I'm totally new to passthrough, AR, MR.

I've managed to get an apk for the passthrough with the default VR scene in unreal. but other than that everything else is kinda vague to me. 

could you please give me a reliable source to get me going for MR development?

meta documentation is kinda messed up and you can easily get lost in it.

thanks