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Seb-wejem
(1) In 5e the hit points and damage output of that opponent will grow. There's a hp/damage-by-level table (well actually a pair of them - the CR table together with the encounter difficulty table).
(2) The reason, in the fiction, that the AC of that enemy grows is because s/he is tougher - her armour is more mythical, her prowess greater, etc. (Ie stuff that, in 5e, is expressed only by growing hp is, in 4e, expressed both by growing hp and growing defences.)
Yeah, that's something I found in 4e, both attack and defences grew at the same rate which made me think that you could have removed the level bonuses, adjusted the monster defence scaling to be half level +X and just have the players use ability modifiers and magic bonuses and it would have played pretty much the same. I thought 4e had some great ideas, I didn't really think the +half level to attack, defences, and skills was one of them. It was like it was added for the sake of adding it because D&D always had increasing attack bonuses.