Hi 

You can add a new stage called "APPROVE" where you can also add a button to approve the request by the manager,after adding the button and the stage you need to define the record rule for the group manager.In the end you can give an attribute called "groups" where you need to specify the record id of the group manager.The below blog would help you to create security group.How to Create Security Groups in Odoo 15

Hope it helps

You need a workflow.
Create a new selection field name it state i.e

state = fields.Selection([('draft', 'Draft'), ('approved', 'Approved'), ('cancel', 'Cancelled')], 'Status', required=True, copy=False, default='draft')

Then create few buttons like Approve, Refuse and actions for the buttons. 
You can study sales workflow in odoo to properly understand the concept.