When the Update button is clicked, Odoo will read the add-ons path configuration variable. For each directory in the list, it will look for immediate subdirectories containing an add-on manifest file, which is a file named __manifest__.py that's stored in the add-on module directory. Odoo reads the manifest, expecting to find a Python dictionary. Unless the manifest contains a key installable set to False, the add-on module metadata is recorded in the database. If the module was already present, the information is updated. If not, a new record is created. If a previously available add-on module is not found, the record is not deleted from the list.