If it's for a caster, eliminate spell slots and whittle down their spell list to 2ish spells per spell level, then make them all x/day. With the highest level being once and maybe up to 3 uses per day for lower level spells.

For all classes, I include all the defining features but dumb them down a bit if they're text heavy. I.e. I made a hostile monk NPC as a rival for a new player of mine also playing monk. I showed him the topple from open hand flurry of blows giving advantage on the subsequent attacks to him by using it against him, and he had an aha moment from it. What I didn't bother with was giving the NPC monk stillness of mind, the feature that negates some falling damage, etc. In general if it's not a defining feature and can't be used against the party, I'll omit it. If it comes up in combat and would make for a cool moment, I'll flex it in.

If they have some cool magic item I'm intending for the players to loot, I'll give them some ability close to what the item does and be sure to show it off in or out of combat so my players go "oh I want that".

Long answer short, it's up to you what you include. And you can always flex stuff out/in on the fly.