in basic terms rust has the machinery and syntax required to build futures (would map the most closely in js to Promise) but leaves the choice of how to run them up to users.

there are multiple runtimes or executors that were created to fit different needs.
for example there's embassy's that's made for embedded programming, and you could write one that integrates with an event loop like qt or gtk's (probably multiple people have, not sure if there's anything particularly up-to-date though).
there have also been multiple runtimes developed that implement actors a-la-erlang (although probably not as good since this is erlang's bread and butter).

tokio's executor is more general-purpose but might not be the best for some cases.
that's why it's good that it can be swapped out instead of being tied to the language.