[2020] Questions about hostiles

I just got this game last night and I'm already hooked but I was confused by my first encounter with a hostile entity.

I got a a dialogue that popped up and some guy (I can't remember the race name because all my brain is giving me right now is the word "Xanax") said "You'll pay for that!" Then it runs right into my base, just opened up my doors, strolled past a few armed adults who happily continued working, and then murdered the only child in my colony.

So this brought up a few questions. First, do regular settlers, like farmers and fishermen, only engage in combat when told to directly or after they've taken their first bit of damage? I played a ton of Rimworld and I was expecting some sort of "stance" setting where I could specify whether a settler would attack on sight, run away, or just ignore the threat but I couldn't find anything like that if it's in the game.

Second, am I missing something with the doors? Do I have to lock them for them somehow to impede enemy movement? Again, the way doors work in rimworld is kind of what I was thinking; open to settlers, friendlies, and neutrals but have to be broken by hostiles.

Third, was it just unlucky RNG that made this hostile entity a veritable baby-seeking missile or do enemies always seek out the weakest members? Is there some way to designate a panic-room to send non-combat people to a safe place until the fighting is over? Or do we have to micro each of our settlers for the duration of combat?