If I have understood your question correctly, for achieving that you would need to define a coordinate system (let's say in the Assembly module Create Datum CSYS) with its origin defined exactly at the point you want to keep as fixed. Then, in the Rotate Instance option in Assembly, select the origin of this newly created coordinate system as the Start Point for the Rotation. This point will stay fixed during the rotation.
Yes, with respect to a coordinate system, you can relatively rotate it. For more, connect with me on my WA https://wa.me/+923440907874, I will guide you in brief.