When you load a scene, you lose variables unless they are saved or in a autoload script, so you would probably change a variable in the autoload script for the scene path of the character chosen in the menu. Then when you load the next scene, it would check the autoload script and load that character scene path and add it to the main scene at location x. There are tutorials for autoload scripts and menu scripts on youtube, just do a search over there naming godot first.