Hello.

If you want to make several changes to your database like adding fields, overriding functions. You have to create your CUSTOM module.

1) If you are adding fields directly on the DB, this is going to work ONLY for that database. Python files and XML files are not going to change if you make changes on the database. Maybe if you reinstall that module, your fields are going to dissapear because they are only in the DB. Module files contain structure and logic of the module, so they are not going to change even if you are creating some fields outside.

2) As I said all the new fields are on the DB, so you can't encapsulate them and move to another database. If you want to, create a custom module.

3) The structure of modules can be different from each other on different ways, if you want to learn what they are doing please refer to Developer Guide. If you want to make some changes on structure it's better to do it on your custom module and not directly on OpenERP modules.