For both cases, we can use TestPublisher, which allows us to programmatically trigger miscellaneous signals:

next(T value) or next(T value, T rest) – send one or more signals to subscribers

emit(T value) – same as next(T) but invokes complete() afterwards

complete() – terminates a source with the complete signal

error(Throwable tr) – terminates a source with an error

flux() – convenient method to wrap a TestPublisher into Flux

mono() – same us flux() but wraps to a Mono 

https://www.baeldung.com/reactive-streams-step-verifier-test-publisher