Isekai is actually a simulation manga.

[zombielunch]

I'm racking my brain trying to figure out a series I had previously read part of but can't find again.

It goes from a fantasy isekai at the start and at the end it turns out to be all a simulation to find people who can fight an invaders (aliens, matrix?) apocalypse

There's a dragon who is the final test for the gate and MC gets to it before he's supposed to be able to by digging faster than the ground can respawn. (He might have visited him multiple times)

I'm also thinking MC was a monster or something... feel like maybe his identity was hidden, but not positive.

Feel like it may have also been into an anime, but can't say for sure.

I've done a bunch of searching but no luck.

 

[Justice88]

Somehow I found out that ender's game also had a manga.
I got not answer for your question, though. It sounded a lot like ender games but Japanese.

 

[hazzack]

maybe this?
villager lv999