I personally will roll “bravery saves” for npcs that are in over their heads, its just a d20, 1-10 flee 11-20 fight, basically just a fight or flight instinct, or I roll a WIS save against the closest players passive intimidate

I roll it anytime an enemy: has its hp reaches a point where they will likely die to one more hit, has no more allies in line of sight, sees an ally die and now has less allies than active threats, is actively intimidated

fight means charging into combat to deal as much damage as possible before death

flight means running away, but it doesn’t necessarily mean giving up on whatever goal they had for ex. If the enemy was attacking in order to steal an item, a fleeing enemy may grab the item as it runs away