Clarity about Armour

First time providing feedback and hope I'm in the right place.

I'm wondering if its possible to clear up how mechanics like Armour work in the game.
From my understanding and research, the value shown in-game does not reflect actual physical damage reduced and looking online for info made me even more confused.

I think looking online for info about something as basic (sorry if its not) as a defensive stat should be a thing (imagine looking up how the 75% elemental resistances work).

I had the same issue with the previous way evasion was calculated but thankfully it was changed to a more understandable format.

In general the game has a lot of info that isn't available or explained anywhere other then online and it would REALLY help new players like I was (Path of Exile 2 was my first actual dive into ARPGs such as these).