How is this ability working under the hood? I see the indicator on my opponent indicating what type of shield I'll be getting, but how is it properly determined? I've been trying to learn her but the indicator will sometimes switch on a champ and it's kind of confusing.
It's adaptative (remember this ter it will be used in future runes)
I can't find the full explanation but to be short it will automatically figure out what are the most damage done by the champion. Let's take Teemo as en example if he took Q first and started 
. if he trade level 1 it will consider that he will deal more physical damage.
But once he take E and get let's say a 
after a few hits (over minions or whatever) the shield will see that he deals more magic than physical and will adapt accordingly.
It gives her an AD shield if the opponent has more AD then AP
It gives her an AP shield if the opponent has more AP then AD
Usually a lot of champions start the game with giving an AD shield because even AP champions have more base AD then AP and scale up with leveling and items. There's a few internal rules to this we're not sure of, but my guess is unless something is obviously building AD, the AP value for the shield is increased so if they obviously have AP runes or items it will outweigh their AD. (example of my theory: AP shield = if AP value x 3 > AD value) Completely unconfirmed.
IIRC, it takes into account your own mr and armor compared to how much magic and physical damage the enemy champ has done throughout the match, and adjusts her passive accordingly. Just because 
is supposed to do magic damage, the full crit version will give you an armor shield
Depends how much damage of each type the champion has done to you. If they've done more physical damage than magic (throughout the entire game), you get an AD shield, if they do more magic damage than physical, you get a magic damage shield.
If it changes, it might be because that champion had been trading with auto attacks a lot, and thus a lot of your damage had been physical, but then they might've started getting more AP and their abilities might've started doing more damage, so you'd taken more AP damage than AD, so then you get a magic shield.
