When data is added to your app - usually via an Add Form - the data will initially be added to the Glide Table that the form screen is directed at, and then will sync with the connected data source (if there is one). So in your case, the data would first be written to a Glide Table, and would then sync with your connected Google Spreadsheet.
If you wanted to, you could also trigger a webhook at the same time that could send the same data to Integromat (or where-ever). Although that step I think would probably be redundant based on your use case description.
Assuming that will you require your Customers to sign in to your app, then each customer will have their own row in a dedicated User Profiles table, and this rewards value could be an attribute (column) in that table. As long as a customer is signed in, that value would be available to use anywhere in your app.