If I config mongo with nest js its come like this,

my files like this,
app.module.ts
item.schema.ts
items.module.ts
items.service.ts
If I config mongo with nest js its come like this,

my files like this,
app.module.ts
item.schema.ts
items.module.ts
items.service.ts
I use a forwardRef in the other modules where I declare the mongo models like this:
@Module({
imports: [
forwardRef(() => AppModule),
MongooseModule.forFeature([{ name: 'User', schema: UserSchema }])
]
})
Where AppModule my app.module is
Just remove the ItemsService from the AppModule and exports it on your ItemsModule, besides no need to define the ItemsController on the AppModule it's enough to be defined on the ItemsModule.
You may have a look about contollers and exports following this link https://docs.nestjs.com/modules