>>713883763
The requirement in order to unlock the extra stage on a Touhou game, varies depending of the entry. In this case, on EoSD/Touhou 6, you have to 1cc (One Credit Clear) the game on Normal or higher; in other words, you have to complete the entire game without running out of lives or getting a Game Over once. Once you have completed that, the extra stage will be unlocked only for the shot type you used to play the main game; that is, if you want to unlock the extra stage with the other shots, you have to 1cc the main game with the other shots again.

Or you can use the unlock code and unlock everything. Whichever way you choose, the main point is to play the extra stage, so use whichever way suits you best.

https://maribelhearn.com/faq/extra
https://en.touhouwiki.net/wiki/List_of_Unlock_Codes