That's correct - none of those 'stack'
What each of them are doing is giving you an AC calculation method; a way of calculating your AC. You choose ONE way of calculating your AC, and that is what your AC will be. So, if you are a Multiclass Barbarian/Monk/Dragon-Blood-Sorcerer with barkskin and mage armour on yourself, you have a number of different way that you can calculate your AC - you could use Monk Unarmoured Defence, and your AC would be (10 + Dex + Wisdom), OR you could use your Barbarian Unarmoured Defence (which it would have been less confusing for them to name something different), to have an AC of (10 + Con + Dex, shield permitted), OR you could use your draconic bloodline method, being (13 +Dex), OR you could use Barkskin's method, which would make your Ac (16, flat, if and only if your final AC would be lower than 16). Mage Armour, too, gives you another option for how to calculate your ac - (13 + Dex), even though that's the same as the sorcerous bloodline method.
Each of the methods are independent ways of getting an AC; they cannot be added together. You can have lots of different ways of calculating your Ac, you just pick which one you use - generally speaking, you'll always use whichever yields the highest AC at the time.
(Edit for clarity: Shields are usually a separate thing, and add their Ac bonus onto whatever base method for calculating that you choose, the same as any other discrete Ac bonus, such as a cloak of protection. Some base AC calculation methods specifically call out not being allowed shields, however (Monk's unarmoured defence is like this, while Barbarina's is not). Shields are wielded equipment, not armour, so they don't interfere with Mage Armour.
Last edited by Niara; 22/09/22 12:19 PM.