How do I get an object inside a model by only knowing the first part of its name?

This seemed fun so I thought I'd answer it.

1. I recommend you use the server storage for each of your maps.
2. I recommend you use a folder called Maps
3. I recommend your maps be grouped into a model.
4. I recommend you clone the map and set its parent to the workspace to spawn it. It will spawn directly where you built it.

This script will be made with the above in mind.

local storage = game:GetService("ReplicatedStorage")

function SpawnMap(MapNum)
    for _, Maps in pairs(storage:GetChildren()) do
            local MapToSpawn = "Map" .. MapNum  
        if Maps.Name ==  MapToSpawn then
            Maps:Clone().Parent = workspace
            local MapObjects = Maps:GetChildren()
            for _, MapObject in pairs(MapObjects) do
                if MapObject.Name == "PlayerStartDoor" then
                    local PlayerStartDoor = MapObject
                    PlayerStartDoor.Lock = true -- Door is now locked
                end
            end
        end
    end
end

This scripts spawn a map of your choosing.

SpawnMap(1)

This will spawn Map1

when Map1 spawns the script will look for the door and lock it.

Loops are fun.