Start here. This is the direct spoken answer to practice first.
Overview
Routing is a product and reliability policy, not a random choice of the cheapest available model.
I route from trusted application context such as task type, required capability, risk, region, latency target, and tenant policy. Each route names an evaluated model and configuration that is known to satisfy that task. The selected route is recorded on the trace so quality, cost, and failures can be compared accurately.