The Database class ties entities and DAOs together:

@Database(entities = [NoteEntity::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun noteDao(): NoteDao

    companion object {
        @Volatile private var INSTANCE: AppDatabase? = null

        fun getInstance(context: Context): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "app_database"
                ).build().also { INSTANCE = it }
            }
        }
    }
}

This is the classic thread-safe singleton pattern: @Volatile ensures visibility of the field across threads, and the synchronized block prevents two threads from both creating a database instance simultaneously. You only ever want one AppDatabase instance per app process.

When your schema changes later (adding a column, a new table), you must bump version and supply a Migration — Room will crash on launch if it detects a schema mismatch without one, by design, to prevent silent data corruption.