The people in his world can get skills by three medium
First- People are born with the skill.
Secondly - If the world acknowledges them they get a skill.
Acknowledgement can be counted- when you break through or do something... Etc.
Third- by learning from a book, called a skill book. It has random skills. When you open it you get a skill. Manly found in the dungeon chest.
There are also scrolls - Which you can read and practice.
Ultimate Skills- Mostly, people are born with this skill.
It's very rare to have an ultimate skill.
Unique skills - People can get it through world acknowledgement or skill books. (Also some rare scrolls)
Skills - By doing some action repeatedly. You can get a skill. And also by acknowledgement or skill books and scrolls.
For example- By jumping 100 times, you get a jump skill, it makes it easier to jump ( something like that)
Level up.
To level up a skill. Certain requirements must be fulfilled, there's also another way by getting a unique stone which is very rare.
...
Please tell me if I am forgetting something or if you have any questions.