How exactly do Skills and Perks work?

So before going into KCD2 I figured that I'd finally play KCD from my pile of shame.

And heck have I been missing out!

But one Question remains: how do skills and perks work? To be precise: How do I aquire them?

For example: I am Lockpick Level 4 but cant unlock "Lasting Lockpicks" which it tells me is a Leveln 3 requirement.

Same goes for different Skills.