Functional core usually means pure functional functions, aka the return value is know if the arguments is known, no side effects required. All the side effects is then pushed up the imperative shell.

You’ll find usually that side effect in imperative actions is usually tied to the dependencies (database, storage, ui, network connections). It can be quite easy to isolate those dependencies then.

It’s ok to have several layers of core. But usually, it’s quite easy to have the actual dependency tree with interfaces and have the implementation as leaves for each node. But the actual benefits is very easy testing and validation. Also fast feedback due to only unit tests is needed for your business logic.