Please have a look at these posts in this OptiX forum and follow the links inside them.
This describes the main differences:
https://forums.developer.nvidia.com/t/porting-to-optix-7/79249
This contains most of the required documentation and example resources to answer your OptiX 7 questions:
https://forums.developer.nvidia.com/t/optix-7-1-release/139962
I’ve explicitly ported my OptiX 5/6 based advanced examples to OptiX 7 to show that process:
https://forums.developer.nvidia.com/t/optix-advanced-samples-on-github/48410/4
Here is some more information:
https://forums.developer.nvidia.com/t/optix-7-and-msvs-2017-hello-world-on-windows-10/141711