Can someone explain why a one way door needs 2 parts to it?

You don't need two bricks.

script.Parent.Touched:connect(function(hit)
    if hit.Parent and game.Players:GetPlayerFromCharacter(hit.Parent) then
        local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
        local char = plr.Character
        local tor = char.Torso
        tor.Velocity=script.Parent.CFrame.lookVector * 100
    end
end)

'Can "Instance.new " be used to change maps for a game, if no then what is used to make a map change'

Instance.new() creates objects, anything you can see in the object browser. You can use InsertService to insert maps and other models that you or the user ROBLOX own.

The common one way door require 2 part since the first part serves as the trigger for itself and the second part which is the blocker. The blocker will prevent you to activate or go in the door once more.

Instance.new is used to create an object using its class name. You may use the ServerStorage to store a map and clone it into the workspace with a script.

I hope that helps!