A lot of it depends on the complexity and the technology stack. Microservices based architecture with massive amounts of microservices and other service depend

A lot of it depends on the complexity and the technology stack. Microservices based architecture with massive amounts of microservices and other service dependencies, its a pain to do more than local unit testing and mocks. Are you using FaaS or a lot of other cloud features? Also difficult to fully test locally. Extract as much logic out of the cloud dependant code. Often there will be sub-sections of large apps that can be run locally. And if the only way to test is in production hopefully they practice some form of limited rollout so you can control who adopts the changes or similarly some form of optional feature flags that are used to enable the new behavior until it can become a new default. If their release model is "We are so good it goes live for everyone all at once" then that tends to make for a stressful release days.