Trying to make a selection gui to select stuff in workspace?

the example is like Miners Haven where you can select multiple parts i thought it was a gui and position and size according to the mouse but this doesn't work

local player = game.Players.LocalPlayer	
local mouse = player:GetMouse()
local gui = player.PlayerGui:WaitForChild("ScreenGui").Frame

mouse.Button1Down:Connect(function(down)
	
	local MouseCFrame = mouse.Hit.p	
		gui.Position =UDim2.new(MouseCFrame)
		
		mouse.Move:Connect(function()
		gui.Size =UDim2.new(mouse.Hit.p)		
		end)
end)

might be doing it wrong, i haven't scripted in a while.

local selectionbox = Instance.new("SelectionBox")
selectionbox.Parent = game:GetService('Players').LocalPlayer.PlayerGui
local mouse = game.Players.LocalPlayer:GetMouse()


mouse.Move:Connect(function() --Detects Mouse Move
	local part = mouse.Target --Current mouse Target
	selectionbox.Adornee = mouse.Target --Changes selection box's Adornee to current Target
end) --end for {mouse.Move:Connect(function()}

There is an Instance called SelectionBox just use that.