How to code some sort of road building system, like Sim City or Skylines?

Even though this is kind of a request I will try to give you hints to make this. REMEMBER THAT THIS IS NOT FOR FE, THIS IS JUST AN EXAMPLE

First, make a model of something and then use the mouse .Move event.

local model = workspace.Model
local mouse = game.Players.LocalPlayer:GetMouse()

mouse.Move:Connect(function()
	if mouse.Hit.p ~= nil then
		model:MoveTo(mouse.Hit.p + Vector3.new(0,5,0))
		-- or you can use this
		model:SetPrimaryPartCFrame(mouse.Hit.p + + Vector3.new(0,5,0))
	end
end)

And for the clicking, there are many ways to do this, in this case I will clone it.

mouse.Button1Down:Connect(function()
	local new = model:Clone()
	new.Parent = workspace -- or anywhere you want
	new:MoveTo(mouse.Hit.p + Vector3.new(0,5,0))
	-- or
	new:SetPrimaryPartCFrame(mouse.Hit.p + + Vector3.new(0,5,0))
end)

If you want the model to be transparent before placing then, do it yourself, it's easy, use for loops to scan through parts

For the other things, try to search it yourself or learn from a free model