In order to access the entities (such as tables and pages) and functions in a Business Central application, you need to have the symbols for that application. Remember that the symbol file is what contains the metadata about those entities so that Visual Studio Code knows what is accessible.

You get the symbols by installing the application to your development sandbox and then using Visual Studio Code to download them to your AL project. After you have obtained the symbols, you need to tell your application to use them, and that's done by creating a dependency entry in the app.json file of your AL project.

When you have done all of this, you will be able to extend entities and use any available functions in another application.