How can we actually create interactive doors, things in general?

Use magnitude

Example:

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

if (char.Torso.Position - workspace.Door.Position).magnitude >=5 then
    player.PlayerGui.ScreenGui.Frame.Visible = true
end

To make the door open after this GUI is shown, simply use UserInputService

And whenever the key to open the door (let's say E), then check if the Frame is visible, and if it is, you can then create your own things.

I'll teach you how this service works, so you know next time, it helps a lot.

We'll start by obtaining this service.

local UIS = game:GetService("UserInputService")

function onKeyPress(key)
    if key.KeyCode == Enum.KeyCode.E and player.PlayerGui.ScreenGui.Frame.Visible == true then
        OpenDoor() -- Link to the open door function, which has your own code to open the door.
    end
end

UIS.InputBegan:connect(onKeyPress)

Create your own OpenDoor function like this:

function OpenDoor()
    -- code to open door here
end

-- or

local OpenDoor = function()
    -- code to open door here
end

Hope I helped! If you are confused about ANYTHING regarding GUI's, UserInputService, I will be more than happy to provide links or information.

Also PLEASE accept my answer, if it helped you, or if it was right.