How exactly do remote functions work?

I know how to use remote events but not remote functions. I made a Local Script that makes it so that when i press a button, i use :InvokeServer() and on the Server Script that's receiving the invoke, a part is created. I tried to return the part so that my Local Script has access to it but i can't seem to do it.

local UIS = game:GetService("UserInputService")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteFunction = ReplicatedStorage:WaitForChild("RemoteFunction")

local player = game.Players.LocalPlayer
local char = player.Character
local humanoid = char.Humanoid

UIS.InputBegan:Connect(function(input)
	if input.KeyCode == Enum.KeyCode.G then
		RemoteFunction:InvokeServer()
	end
end)

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteFunction = ReplicatedStorage:WaitForChild("RemoteFunction")

function RemoteFunction.OnServerInvoke(player)
	return local Box = Instance.new("Part",game.Workspace)
	Box.Transparency = 0.5
	Box.CanCollide = false
	Box.Anchored = false
	Box.Size = Vector3.new(19.29, 0.87, 2)
	Box.Name = "Box"
end

Remote functions have different syntax compared to remote functions. OnServerInvoke is not called as an event, but with an assignment operator, since Remote Functions return a specific value back to the client. A framework for your code would look like so:

--Localscript
UIS.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.G then
        local returnValue = RemoteFunction:InvokeServer() --InvokeServer will return something, which is then stored in the returnValue variable
    end
end)

--Serverscript
function doSomething(player)
    local Box = Instance.new("Part",game.Workspace)
    Box.Transparency = 0.5
    Box.CanCollide = false
    Box.Anchored = false
    Box.Size = Vector3.new(19.29, 0.87, 2)
    Box.Name = "Box"
    return Box;
end

RemoteFunction.OnServerInvoke = doSomething --Use the assignment operator to bind the function

Hope that helped