Well, it varies. In 'Re:Zero − Starting Life in Another World', Subaru levels up his skills based on his repeated experiences. Every time he dies and restarts, he can learn from his past mistakes and grow in terms of abilities. In 'The Rising of the Shield Hero', the shield hero levels up by fighting enemies and also by finding and using rare items. His shield evolves as he levels up, which gives him new powers. In 'Log Horizon', the leveling system is based on the game mechanics they are familiar with, but with some twists due to being trapped in the game world. They level up by completing quests, fighting mobs, and learning new skills within the game world.
Well, in many such novels, it often involves defeating enemies. For example, in 'Solo Leveling', the protagonist gets experience points from killing monsters in dungeons. Each kill or completion of a dungeon level gives him points to level up his stats like strength, speed, etc.
In many 'super leveling system novel', it often works by the character achieving certain goals like defeating enemies. For example, in 'Solo Leveling', the main character gets experience points from hunting monsters, and as he accumulates enough points, he levels up and gains new abilities.
Solo Leveling is definitely a manhwa. Manhwa have their own style and fan base. Solo Leveling stands out among them with its fantasy elements and exciting adventures that have attracted readers worldwide.