Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
App Service, AKS, and Azure Functions offer different control, scaling, and ownership boundaries. The right host follows the workload and the team's ability to operate it, not the novelty of the platform.
I would start with App Service for a normal .NET web API when the team wants managed hosting, simple deployment, scaling, TLS, app settings, and diagnostics without owning Kubernetes. I would consider Azure Functions for event-driven or scheduled work where the unit is a trigger, not a long-running web app. I would reach for AKS when we need container orchestration, custom networking, sidecars, or platform-level control that App Service does not give us.