I guess the same way as aurora serverless. In general, RDS uses a seperate storage layer from the actual instances so you can do vertical scale/upgrades with 0 downtime (either read replica goes down or replica becomes master)

We run 3 nodes 24/7 (writer + 2 readers) and have RIs for them. But during daytime hours we autoscale and run an addition 8 or 9 readers. Some of these run for just a few hours and could easily run on spot (especially with minimum duration).

Serverless couldn't provide enough capacity for us (at peak we use up to 300 vCPUs in this cluster). That was on v1. v2 might change that when it supports postgres support.