[charcircuit]

That is an implementation detail on where you put the code that is blocking or running concurrently from the main code. An executor could use a separate OS thread, or the application could itself schedule application levels threads onto a number of OS threads.

When writing a Future that will block for 5 seconds you will need to find somewhere to that you can put the code to block for 5 seconds. You don't technically need to even use an executor here.