Hi, I'm trying to customize the odoo POS (Point of Sale) app (odoo 14) via code, so I tried to find some tutorials but they are all very shallow and only focus on the "customization" via web (like... changing titles, payment methods). I would like to know if there is any procedure I need to make to not brick the already existing codes. For exemple, I would like to change the page that is called when you press the "payment" button, I would also like to change the way items are added to the order, instead of clicking in it I would like 2 buttons beneath the item, a " - " and a " + " symbol, maybe even change the first page of the POS, instead of calling the order+item(productscreen) page call some page with some greeting text with a button "start" and then going to the order (productscreen). 

These kind of customizations is what I am looking for and I couldn't find any tutorials, so if someone could aid me with this, maybe point me in the right direction, I would really aprecciate that.