(Java) Relationship of Reactive Stream and Reactor Projects
The reactivestreams project define the standard APIs in this field:
Here are the core interfaces:

The above interfaces are the core structures defined in org.reactivestreams.
Another popular project, spring-reactor, implements the above standard project:
In the project, there are two important classes, Flux and Mono, implements CorePublisher, and CorePublisher implements Publisher interface from org.reactivestream:

From above we can see both Flux and Mono are implementations of Publisher.
In JBoss Mutiny, it has support to the reactor project:
In above module of Mutiny, it provides a series of helpers to convert Reactor classes:

From above we can see the adapter provides multiple methods to convert Flux and Mono to Mutiny classes.
In addition RxJava also provides adaptor like this:
Which I won’t explore in this post.