I am not sure I understood, but if I got it correctly, you have the same problem I got with godot some while back, where you dont have a way to change a scene passing a parameter : https://godot.community/topic/63/set-start-parameter-to-packedscene/7
The current solution that everybody uses is like zini says, use globals... I really hate using globals for that, but there is another way, that is changing the way godot works and work with your game using this new way of thinking :
1) Create an empty main root scene, that will be allways active, then extend it so you can uses this on everything. In this scene you have a changeMainSceneWithData function that accepts a parameter, and use this parameter to send info about stuff you want to send to the new scene, like player data etc.