Looking behind the curtain (Score 1)

A lot of extensions request access to your browser's X, Y, & Z... and sometimes your entire file system (???) But since we (the user/s) wants to use the provided functionality in the extension, we all click "OK". Just from reading those notifications, it is still unclear WHY the extension needs those access permissions, or WHAT the extension might be doing with said access. How can we know/understand more about this process? Where is the source path of the extension & should we just be looking at the source code (assuming dev experience)?