Finding the target model

I want to allow players in-game to select models and parts, similar to how we do in studio, and then I want the server to know that part/model is selected by that player.

How could I make this? Where do I start?

(like an RTS game)

On the client, you would want to check what the mouse’s target is, and then if it is an applicable target have the client fire a remote event to the server telling it what model you are on. A bit of an oversimplification but I hope it puts you on the right track. If you have any issues you can dm me or reply here

Target returns the current part/model the mouse is on? So its not like Hit?

Yeah, Target is the part that the mouse is currently on while Hit is the CFrame of the mouse in world space.
So you would just check if the target is one of the interactable objects and then fire your remote

The Mouse Object has been superseded so I suggest you use an alternative like
Its basically the same but using UserInputService