How to set up a sales screen

Could someone help me configure these functions

my database

One way to go is have an app variable which will hold the current items in the cart. The list will be repeated with the cart variable.

As for adding the product id and quantity you need to get the data from xano,

Look for item details based on the id.

Add the item to the variable cart

Total can be calculated easily by adding the prices of the items in cart

do you know if there is any project that shows how to make a shopping cart either on youtube or here on the forum?

I have made a shopping application myself but it is not published yet. You could hop on a zoom call if you want to brainstorm.