So what aspect exactly?
Normally when there is many levels, I see stats implemented via some sort of formulas, which you can play with to get the results you want. Some stats may be linear, others exponential.
stealth_detect_range = 10 + level * 5
hp = 100 + level*level
resist = 0.9 - 0.5/level
You can apply such things to both players and NPC's, and to randomly generated weapons/items. NPC level is selected based on player level, possibly with some random variation, and possibly including difficulty setting.
When there is fairly few levels (e.g. specific skills you spend skill points on, rather than or in addition to a "level" that buffs everything), I tend to see games just explicitly pick values for each level. Sometimes a skill level will have an entirely new effect compared to the one before it.
It mostly just a question of deciding what you want level's/skills to do, then playing it a lot to try and make it balanced/fair.