Once when you defeated the dragon, the exit portal will open.  However, if you set your spawnpoint just above the exit portal and enter it, you will find yourself in an infinite loading terrain loop.  This will force you to close the game with task manager.

Here is a video: https://www.mediafire.com/file/wav8zy1v8qoby2q/Minecraft_1.16.5_-_Singleplayer_2021-03-16_12-38-40.mp4/file