Some information about Interactive Grid I found useful:
The APEX IG Cookbook by John Snyders - an extremely useful set of examples. At this (20.1) time of writing, 19.2 was the latest available version of this cookbook, but you should be able to find the latest version using the link above.
It's called a Cheat Sheet, but Encyclopedia would probably be a better title :-)
Updating Interactive Grid cells.
Interactive Grid Get Selected Rows Example