Answering to your questions:
1 Is it possible for the prescriptions to remove the input fields from the form?
Yes you can remove the fields in the prescriptions form.
Please check out the file.–>> openemr/templates/prescription/general_edit.html
2 What is the table that calls the medications in prescription?
Please check with the openemr table name --> prescriptions
3 Can i import the AIFA Italian drug database into a table?
Yes,You can import the data into openEMR database -> table_name -> drug.
Make sure,whether the importing column fields matches with drug column fields.
If you want to add your own table and fields in OpenEMR database,it needs more customization.
4 Is there a way to keep the changes even after installing update of openemr?
Yes,You can keep the changes even after updating the OpenEMR.
Please check out the files in the folder openemr/sql/.