UsersModule declares UsersService in providers but not in exports. OrdersModule imports UsersModule and injects UsersService into OrdersService. What happens?

Nest throws an error at startup. Providers are encapsulated: importing a module only gives you what that module lists in exports. UsersService is not exported, so Nest stops during bootstrap with a "can't resolve dependencies" error.

"It works, because importing a module gives access to all its providers" is wrong, because importing only exposes exported providers. Nest also never creates a hidden second instance, and it never injects undefined.