Explanation:

https://learn.microsoft.com/en-us/azure/app-service/manage-backup?tabs=portal#create-a-custom-backup In Storage account, select an existing storage account (in the same subscription) or select Create new. Do the same with Container.
https://learn.microsoft.com/en-us/azure/app-service/manage-backup?tabs=portal#configure-partial-backups Partial backups are supported for custom backups (not for automatic backups). Sometimes you don't want to back up everything on your app. To exclude folders and files from being stored in your future backups, create a _backup.filter file in the %HOME%\site\wwwroot folder of your app. Specify the list of files and folders you want to exclude in this file.