I'm looking for a manhwa where the main character gets a system in a modern world where cultivation is a thing to the entire world.
He gets a system to learn cultivation techniques and he learns them while fighting monsters from the modern world but where he trains is in his mind, and it increases his power the stronger the monster is the stronger he becomes.
The querent posted a matching work in the comments, Level Up in the Mirror by Runaway loli:
My name is Feng Yun. No cultivation resources, penniless? It doesn’t matter, I have a mirror, as long as it shines, whether you are human or beast, whether you are dead or alive, all things are projected out. What martial arts techniques, what heavenly treasures, what weapons, even talent qualifications are all projected for you!
The basic idea behind it is that Feng Yun has been isekaied into a world of cultivation, but has no natural talent until his cheat skill kicks in, tied to a mirror necklace he wears, which allows him to take someone reflected in the necklace, and fight them virtually inside his mind, gaining "drops" of their abilities if he defeats them.
