Beginner Help Needed with Robotic Manipulator

Hello Everyone, I am writing this after couple of months.

Here are different cases, where I am completely stuck.
If I would want to add external 3d modules (with .usd extensions) and the robot being imported with URDF importer. How can I attach them together? In my case, if I’d “Snap” them and make them “Fixed Joint”, they don’t work. I am a beginner, however I am finding Isaac Sim very difficult to understand.
The next big milestone will be to move the robot in Isaac Sim, seems like I will never be able to do it.

Can the mods, share easy to digest beginner guides? The docs section seems heavily written with no clear direction and usage.

Thanks