In Salesforce B2C Commerce Cloud, when multiple cartridges are involved, the cartridge path determines the order in which cartridges are evaluated. If a developer adds a custom extension to a route in a cartridge that is listed before the original cartridge in the cartridge path, the custom code will execute first. Assuming that both the original and custom implementations of the route are correctly implemented without overriding each other completely, the custom code executes, followed by the execution of the base code. This order of execution is due to the way Salesforce Commerce Cloud handles cartridge paths, where cartridges earlier in the path have higher priority.