How does Magic Pen work exactly(and armor pen I suppose)

so first off there is magic shred/reduction which effects the target's MR and will make your teams damage to them go up as well since they effectively have less armor/MR and then there is magic penatration which effects the damage one unit is doing to another unit

There is also flat and % of both and the order is The order is flat MR reduction % MR reduction % MR penetration Flat MR penetration

so 100 MR will get reduced by abyssal scepter by 20 MR making it 80 then that 80 becomes 68 MR after kayles passive fully stacked reduces it by 15% next is void staf which penatrats 35% of that 68 leavign you with an effective 44.2 MR Last is flat penetration which if you have say a Haunting Guise will make it 29.2 MR

This works the same for armor as well

Now how armor and MR works is you have (100/(100+X)) X stands for the ammount of armor or MR you have so 100 MR makes it so that you take 50% magic damage Or in the case after the reduction and pen example I gave earleri leaving you with an effective 29.2 MR you take 77.399% damage.

and while it might seem like it matters less having higher armor and MR your effective health against that kind of damage is increased by 1% for every armor/MR you have so a tank with 4000 health and 300 armor has an effective 16,000 health before reduction and penetration.

I hope that answers the question

Yeah.