It’s not an easy task depending on the setup. I can at least help direct the topic. Are you talking about 2D or 3D.
Are you saving a single setup or multiple versions with names or slots? A single setup saved to storage is much easier than multiple slots or external Json files.
Varauables are relativly easy to save to storage especially if they are setup inside a single structure variable.
Objects are realitivly easy to save and load using a group and saving and loading as a Json string using an array of structures that represent the objects and the desired elements like positions and animations or object names. The number of elements you use related to each object is highly customizable but requires a decent understanding of arrays and structures.
This is a simple version using trees. It creates the same object and just changes the animation. If the object name is saved then the object can be created from a group using the name.
This just a simple example.
https://forum.gdevelop.io/t/how-to-save-a-games-world/54560/8?u=keith_1357
There is no single save game action. Each situation is unique.