The limit is "being set" in the Player.gd script, on the "_ready" method:

func _ready():
    screen_size = get_viewport_rect().size

The Viewport is the screen that contains the game, so it's size is the same as the game window (sorta, mostly). That's where the 720x480 comes from.

They are then being enforced when you move the player:

position += velocity*delta
position.x = clamp(position.x, 0, screen_size.x)
position.y = clamp(position.y, 0, screen_size.y)

Looking at the documentation for "clamp" (by ctrl+clicking the function), you'll see it takes a value (position.x) and ensures it's within a set range (from 0 to either screen_size's y or x component).

Dodge the Creeps is an interesting tutorial in the sense that it's very different from the way you'll usually make beginner games in Godot (e.g.: using KinematicBody2D for the player and moving with move_and_slide), so it might not actually be the best introduction (we really could make it better for 4.0 release, that'd be cool). I'd recommend watching some tutorials from YT by such as GDQuest, KidsCanCode, Heartbeast and GameEndeavor to get the hang of it and then diving into code samples and the documentation's articles later to learn best practices and coding patterns. If you deeply dislike YT videos for some reason, then check out the KinematicBody2D, StaticBody2D and Camera2D nodes.

Edit: I've just checked out your asset pack and you'll probably need a TileMap node too.