The files located in Odoo/Odoo/Addons/Base/Res but you should not make any changes in original Odoo files. you should make your own model and do everything trough inheritince

Hi you can get which model and xml_id you need to inherit easily by following this step:

- activate developer mode (add ?debug= after web/ in url)

- hover mouse pointer over a field in form view, it will show details of that field including its model (for contacts model is res.partner)

- there will be a dropdown for debug mode on top right corner of your screen, click on it and select edit form view, from there you will be able to get the xml id of the form