>giving class levels to npcs

generally in established settings actual genuine class levels are extremely rare
a king could have an army and only one of the soldiers may be a level 1 fighter
a class level isn't something you just add to something to bring it on par with pcs, adding more stats does that much more effectively
a class level is more of a title, something that separates the masses of commoners and unclassed with the main characters of a multiverse
having a class level means you're somebody, somebody with the ability to change fate, events, even history

class levels represent potential moreso than capability

1st level means you have great potential
one day, you can become 20th level, maybe even beyond that

take into account the significance of experience as well
towns and cities are populated by guardsmen who have worked it their entire lives, yet their stats are generally no better than the cr1/4 guard or cr3 knight, why?
because adventurers are adventurers
adventurers don't have the same daily life a normal man has, they don't clock in for the day, strapping on their armor and repeat the same daily routine before heading back for the same daily rest
they go out and see sights, perform acts, uncover secrets, life or death situations and battle monsters forgotten by time, this is experience
living a life on the edge where one mistep could be their death, they accumulate experience far faster than a normal man
this is why an adventurer eventually becomes stronger than the captain of the guard who's been doing his job for 20 years in say, just one year

class levels and experience actually exist in-universe, in a sort of meta way, if you know what i mean

remember a commoner has 4 hp
adventuring is a lifestyle not chosen just because yolo
the most popular reasons are generally fame fortune and power, but even then, it's a suicidally dangerous profession for what amounts to a literal get rich quick scheme

the class level is a title, sign of potential and capability
you can always add second wind to a guards statblock
but if the guard in particular is a main character of the story of the world, a potential driving force who can have an affect on things, that would be what a class level represents

the conquerer king of a rival kingdom could be a warlord statblock, but if he's a real mean green meme machine, one with the capability to actually do something historical like start a new globe spanning empire, maybe he's actually a level 10 fighter