Most of the YouTube Tutorials are a good source of learning How to use the Ark DevKit. As to your specific issues, Pick Up rocks are called “Small Rock”, search for Small Rock in the Content Browser and their should be 3 or 4 popping.
There is also a Video with a Basic MapKit. I can´t find it right now but here´s a link. Basic_MapKit - Google Drive
Simply create a new folder under mod/YourModName and drag them into the new folder. Launch ADK load your new mod/map. Under Matinee - Matinee_DayCycle you can go through Day/Night by cycling thru the timeline on the bottom.
Grass etc. like dreyn74 posted can be applied to a texture through a Clutter Layer. Every texture you piant on your landscape has a designated layer eg. Grass has a designated layer number 1.
To find out which layer has which number, find the layer assigned in the content browser, double click on it and you´ll see the assigned number.
Here´s the Tutorial made by BrickWhut https://youtube.com/watch?v=okpWBqoA1oM
For Spawning NPC/Dinos this is helpfulhttps://youtube.com/watch?v=w3ZNVxtUArg
But as I mentioned YouTube is a great source of learning the basics. Some is Outdated (Weather/Day/NightCycle)
It is also recommended to use the Unreal Engine Documentation to help you understand and learn.