In this world, everyone is born with just one skill, and that skill will decide their entire lives. It is simply based on the fact that those without skills can never match those with skills in the same domain.
In short, even if you practiced blacksmithing for forty years, someone with a Master skill in blacksmithing can reach your level within just a few months of training.
Skills are based on genetics, so they can be passed down, but it isn't guaranteed unless the parents both have similar skills. But the odds aren't that great, with just 10% for both parents' skills to be transferred, and the remaining 80% is random.
As everyone is born with a skill, that skill can be detected by powerful magical tools at birth, but it will naturally reveal itself when the person hits puberty. Controlling the skill can take different amounts of time from one person to another, so it is common for most people to have only mastered their skills by their late teens.
After mastering their skills to the limit, a person can only rank up their skills once. This isn't based on any hard fact or rule, but only on the observation that no one had ever managed to rank up their skill more than once. Even the long-lived elves never managed to rank up their skill twice.
So, if someone is born with a Proficient skill, they can only ever hope to rank it up to Master.
Skills aren't the only path to power, but they are the most accessible to mortals. The other paths include Witchcraft, Arcane Arts, Blessings from powerful creatures like Dragons or Divine Beasts, and even Curses from Devils and Demons. Humanoids can also use monster cores to create magic items to empower themselves.
People usually use their skill as a base and build themselves around it to achieve impressive feats.
Both Skills and all living beings can go through a process called evolution. For skills, it is different than ranking up, as evolution changes the nature of the skill on a fundamental level.
For example [Proficient Arsonist] can evolve into [Proficient Pyromancer].
An example for creatures evolving is a Human turning into High Human, Demonoid, Dragonoid and the likes.
Skill Evolution can be caused by several factors, including traumatic events like having your entire clan wiped out, being tortured for years, or even being forced to make a difficult choice.
Creature Evolution is usually cased by specific circumstances like using too much magic causing someone to slowly turn into a High Human, or being bathed in demonic magic to become a Demonoid. But those evolution usually carry the risk of death, as 999/1000 die without being able to evolve.
The only known way for someone to possess two Skills is to have their twin die in the womb, causing them to inherit both Skills. But they are usually unable to master both skills.