As you know, Thunkable X does not allow offline full data exchange between the WebViewer and the native part. For this you have to use Firebase, which allows you to get fast data exchange. There is also a method of slow data exchange without using Firebase, one of the variants of which is given in the basic example.
https://x.thunkable.com/projects/5bcda40fc3c5f0331502e40a/project/properties/designer