Ok, so what you typically do is to put these different main parts of your game in different collections. So you’d have one collection containing the splash screen (company/game logo etc), one collection for the main menu and then one or more collections holding the game and levels. You can of course decide to divide things in a different way, depending on your game, but an overall structure such as the one I’m suggesting usually works well.
The idea then is to load and unload these collections depending on the state of the game. Collections are loaded and unloaded using collection proxies. I usually have a main collection, that is specified as my bootstrap collection in game.project and this main collection is responsible for loading and unloading the other collections based on game state.
I have an example of this setup in this sample project: https://github.com/britzl/publicexamples/tree/master/examples/menu_and_game
You could perhaps also look into using the Defold UI Routing lib by @megus.sugem.