How, specifically, does the difficulty level affect the game?

I understand what the game already tells you in the general sense... it changes enemy damage, resistances, and chance of legendary monsters appearing.

But, specifically, how does the difficulty level affect the game? Is going from Normal to Hard a 5% increase in damage? 50%? 200%? Are there any additional things changed that aren't mentioned. Does Survival mode add any extra mechanics, or is it just even bigger numbers than Very Hard?

Gameplay changes that affect you as you change difficulty from Easy -> Very Hard -> Survival. I have tested most of this in game in a simple way. I've brought my melee character and have quicksaved before a scripted Swan fight. I found out a few things:

• Damage you deal is affected as you progress (Damage dealt is lowered)
  • On Easy, Swans for me were taking 4 light hits to kill.
  • On Hard, Swans for me were taking 20+ light hits to kill.
• Damage you take is affected as you progress
  • On Easy, Swans could hit me for as low as 12% of my total HP. HP values were the same in both tests
  • On Hard, Swans could insta-kill me at times, like the Giants in Skyrim.
• Spawn rate of Legendary enemies (and thusly 'drop' rates of Legendary gear) is affected as you progress from Easy -> Hard
  • On Easy, Legendary spawns are very "hard" (haha) to come by. Legendary gear is infrequently found.
  • On Hard, Legendary spawns just seem a bit more plentiful. Legendary gear is found more often.
• EXP stays the same. Changing the difficulty did not change the EXP rewards from a quest, or from killing an enemy, or from passing a speech check.
• Explosives become a godsend in Survival mode. The damage is heavily amped up and makes dispatching of tougher enemies like AI turrets and Super Mutants a little bit easier

Survival mode is even worse. Consider Survival mode "Extreme" mode. It features extremely tough enemies, as every single enemy gets huge damage and HP buffs, turning them into deadly bullet sponges. Ammo conservation actually becomes a thing in my survival playthrough, and in terms of playstyle, I found hanging back, slowly going through areas, and backtracking if enemies came a lot more useful. Explosives became very important, as well as mines, which were excellent at covering your ass when a Super Mutant is chasing you down a hallway and your entire clip of Laser Rifle ammo does absolutely nothing.

Multipliers

Difficulty | Dmg Done | Dmg Taken   | Legendary spawn rate
-------------------------------------------------------------
V.easy     | x 2      | x 0.5       | x 0.25
Easy       | x 1.5    | x 0.75      | x 0.5
Normal     | x 1      | x 1         | x 1
Hard       | x 0.75   | x 1.5       | x 1.5
V.Hard     | x 0.5    | x 2         | x 2
Survival   | x 0.75   | x 3         | x 3

Not sure about the # for NPC's HP increase.

It also affect some other things, like the Healing speed/rate, spawn rate of legendary enemies and other similar effects. I don't have hard numbers for those though. There are no new mechanics on higher difficulty.