I have a use case where I have to call two different methods in a reactive pipeline Java 8 on a post-API call.
1st Method:
will insert data in a master table and will return the pk of that table insertion.
2nd Method:
this method will insert data in the mapping table which will use the pk received from 1st method.
I try to do that Mono.zip, but that did not work as Zip is calling both methods simultaneously and is not able to pass method 1 output to 2nd method input.

Call Different Method in reactive pipeline
You can easily do it using the map or flatMap operator, depending on what type of repository you have - reactive or not reactive.
Here is examples for both cases:
public class YourService {
    private final YourRepository repository;
    private final YourReactiveRepository reactiveRepository;

    void doAction() {
        Mono.fromCallable(() -> repository.saveToMainTable("main data"))
            .map(mainTableId -> repository.saveToSecondaryTable(mainTableId, "secondary data"))
            .subscribeOn(Schedulers.boundedElastic())
            .subscribe();
    }

    void doActionWithReactiveRepository() {
        reactiveRepository.saveToMainTable("main data")
            .flatMap(mainTableId -> reactiveRepository.saveToSecondaryTable(mainTableId, "secondary data"))
            .subscribe();
    }

    interface YourRepository {
        int saveToMainTable(String someData);
        boolean saveToSecondaryTable(int mainTableId, String someData);
    }

    interface YourReactiveRepository {
        Mono<Integer> saveToMainTable(String someData);
        Mono<Boolean> saveToSecondaryTable(int mainTableId, String someData);
    }
}

You can read more about map here and about flatMap here
