The following is my understanding after at least a thousand hours of play, so it's likely to be only partially correct (lol). You might want to look up more info.
EHP is generally ES or HP(+ MP if you have MoM).
The calc isn't as simple for hybrid builds. ES is less strong in a hybrid build if you don't have good ES regen or a big enough pool for leech to be effective. It will help mitigate the first few moments of an encounter, but you'll generally not have any ES after that first hit because you'll be leeching or regenerating life instead of ES. This makes your real EHP dependent only on your life (and mana if MoM) for the majority of difficult encounters.
Defenses are generally discussed separately from EHP. Your EHP is the absolute amount of damage you can take before you die, defenses are how you mitigate damage before it hits your EHP.
Armor reduces how hard you are hit after the enemy lands the hit, and it's less effective for harder hits. It's in a pretty bad place right now due to how it works on difficult fights or elemental damage.
Evasion reduces the chance an enemy hits you at all. It's an all-or-nothing mitigation that doesn't affect the actual damage you'll take from an attack. Iirc it has a hidden mechanic that, within a short time frame, reduces your effective evasion each time an enemy misses... Meaning that eventually you'll get hit if you give the enemy enough chances. Evasion is pretty strong, especially with Trickster and how you can use it to scale your EHP (ES).
ES is what it says, a shield of energy that blocks damage (except chaos damage) before it hits your health. This is probably the strongest defense, especially when combined with certain ascendancies, passives, and items. It's so ubiquitous and effective that it's generally directly added to EHP, unless you haven't built around it. I expect some nerfs in the near future.
Avoidance/Dodge is another defensive layer, generally available from items, acrobatics keystones, and ascendancies. Dodge is similar to Evasion, but IIRC it doesn't have a "recently dodged reduction" mechanic like Evasion. You roll to dodge, then roll to evade. The mechanics are very powerful together. Combined with Enfeeble and Blind, a large amount of Dodge and Evasion can provide huge defensive gains. However, you'll still get chunked pretty hard if you actually do get hit without other mitigations.
Mind Over Matter is another extremely effective defensive layer. With MoM, 30% of damage is taken from your unreserved Mana pool. Not only do you get more EHP (equal to your unreserved Mana up to 30% of your health pool), but you also get the benefits of Mana regen. Effectively, your Mana regen acts similar to life regen to keep you alive.
