Hello Vsevolod,
I might not have understood this correctly so please let me know if I get my assumption wrong, but if your data is in excel you don't have to create a JSON structure.
You should be using the Excel importer module for this, it also comes with documentation on how to use.
Hope this helps
Update: As Andreas mentioned, there are no videos, but the documentation is found under the Documentation tab (see below), and it's quite thorough
You currently only built the logic to map the data from JSON according to your mapping. You haven't processed any data yet. Now you need to run the application and call this logic with the data to import and process to database objects with this mapping.
You can read more about it here: https://docs.mendix.com/refguide/mapping-documents
But if you just need to import from Excel, you could also use the Excel importer module: https://docs.mendix.com/howto/integration/importing-excel-documents