Hi Mersed,

Usually you do this in Odoo within a configuration menu. This configuration menu is connected to a transient model. The data itself is then saved on the company level, meaning you have one configuration per company (per database).
You can find a good and detailed example in Odoo 12 under the "sales" module at https://github.com/odoo/odoo/blob/12.0/addons/sale/models/res_config_settings.py

In short you have a custom model for your settings like this:

# -*- coding: utf-8 -*-
# Part of Odoo. See LICENSE file for full copyright and licensing details.

from odoo import api, fields, models


class ResConfigSettings(models.TransientModel):
    _inherit = 'res.config.settings'

    sale_note = fields.Text(related='company_id.sale_note', string="Terms & Conditions", readonly=False)

You inherit the res.company model to store your custom settings on it:

# -*- coding: utf-8 -*-
# Part of Odoo. See LICENSE file for full copyright and licensing details.

from odoo import api, fields, models, _


class ResCompany(models.Model):
    _inherit = "res.company"

    sale_note = fields.Text(string='Default Terms and Conditions', translate=True)

You create a view to add your own custom settings:

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="res_config_settings_view_form" model="ir.ui.view">
        <field name="name">res.config.settings.view.form.inherit.your_module</field>
        <field name="model">res.config.settings</field>
        <field name="priority" eval="10"/>
        <field name="inherit_id" ref="base.res_config_settings_view_form" />
        <field name="arch" type="xml">
            <xpath expr="//div[hasclass('settings')]" position="inside">
              <div class="app_settings_block o_not_app" data-string="Your config" string="YourConfig" data-key="your_key">
                  <h2>Your custom config here</h2>
              </div>
            </xpath>
        </field>
    </record>

Regards,
Yenthe