How do you evolve a database schema without any downtime?

were currently facing the challenge of evolving our primary database schema in a live, high-traffic environment. downtime is absolutely not an option. ive been reading about blue-green deployments for applications, but applying that to a stateful database feels trickier. are there established patterns or strategies for managing schema changes, like adding columns, changing data types, with zero user impact? curious to hear what others have successfully implemented.