Checkout customization, Shopify Functions vs Magento module system?

Shopify Plus supports checkout UI extension targets as well as eligible custom Functions apps; it is not limited to a single Functions mechanism. Supported APIs and targets still define the boundary. Magento Open Source and Adobe Commerce PaaS permit application-level PHP changes that your team must maintain. Adobe SaaS uses out-of-process extensibility. Demonstrate the exact discount, delivery or validation rule, including its error behavior, before assuming either extension model can deliver it.

Official references: Shopify checkout customization; Adobe extensibility models.