join is one possible fundamental monadic operation. You can also define monads in other ways, such as endofunctors such that the Kleisli structure forms a category. In this case, you have the identity arrow (called return in Haskell) and Kleisli composition (<=< in Haskell) as fundamental operations. Neither of these is more fundamental than the other. You can convert back and forth,

    f >=> g = \x -> f x >>= g
    m >>= f = (f <=< id) m
    
    join = (>>= id)
    m >>= f = join (fmap f m)

So if you're focused on composing monadic operations, thinking about it in terms of >=> is often a good idea.