How do yall get a lot of ES?

Hey, i'm confused about how people reach very high energy shield amounts.
I was reading a summoner guide that said that 10K es was fairly easy to get, much easier than 10K life, i'm currently sitting at 2.3KES and while i can see how to get to 3 or 3.5K i really don't see how 10K is reachable.
Am i missing something obvious? Some huge +100% ES node or a unique i don't know of? I currently have a 600ES chest & while i know i can get a 750ish one & prolly 300 more on the other gear it just doesn't add up to 10K
Any help welcome!

Getting around 100% or more increased energy shield (combining Intelligence, passives, and possibly amulets) should be fairly easy to get. You can easily get 40% increased ES from having 200 Intelligence, and then you can cover the rest with ES nodes.

Then, when your base ES is roughly doubled, each piece of flat ES gear really adds up.

Suppose you have a 600 ES chest (which is pretty dang good in my opinion). Then, maybe an average of 150 each on your 2x2 pieces for a total of 450 base ES. Then there's your shield, which could grant anywhere from 100 to 200 ES depending on build and richness. Then, if you're really serious about stacking ES, you get some here and there on your jewelry and belts - let's say an average of 15 each to be conservative. That's 60 base ES. Now let's plop a Discipline aura on top; let's say level 18 to be conservative again.

600 + 450 + 100 + 288 = 1438 base ES in a rather most conservative case. Then, with 100% or more Increased Energy Shield, that will take you to 2876.

600 + 450 + 200 + 288 + 60 = 1598 base ES. That's 3186 total ES.

Now, if you really want to start cooking with petrol, you can stack more Intelligence on your gear and also grab Chaos Inoculation, for some Increased and More shield. Let's say you get 140% increased Energy Shield all in all - pretty reasonable considering I've already broken 100% inc on my LLRF Scion with way not enough nodes (and no CI cluster because LL). Then you have 12% more energy shield on top of that. Let's take our base ES case from before, approximately:

1598 * 2.4 * 1.12 = 4295 total ES. Nice.

As for things like, well, 10k, that's probably the province of the very rich.

There are a significant handful of ES chests that are 900+ showing on poe.xyz.is. Similarly, 250+ boots, 250+ gloves, 450+ helmets, and 600+ shields. Moreover, we can be generous with our jewelry now, since we are rich. 40+ on each should be trivial, for a total of 160 or so.

On top of that, let's stack a level, oh, say, 21 Discipline for 333 ES, and multiply it by maybe 34% bonus for increased aura nodes. 446 base ES in the end from Discipline.

Nodes? Well, the ES wheel above Scion grants 60%. Witch clusterlet grants about 40%. Shadow hybrid clusters each grant 32% and 40% respectively. Let's say we only complete three out of four of these, because speccing only ES is boring. There's about 140% increased already, without taking into account Intelligence, which should provide at least 50% additional increased, and without taking into account the CI cluster from before. I'd venture to say 300 INT, or 60% inc ES, is reasonable. Let's say our total inc ES is about 218%.

So let's take a percursory calc of this.

(900 + 250 + 250 + 450 + 600 + 160 + 446) * 3.18 * 1.12 = 10884 total ES.

Look at that, then! Already broken 10,000 ES and we don't even have best in slot gear. Close, yes. But not perfect.

EDIT: Someone once calculated the theoretical maximum for Energy Shield to be somewhere above 20,000. Not sure if it's still possible, since I think ES nodes were nerfed back then. And honestly, who would want to build ONLY ES? Some damage or utility nodes/items would be nice every once in a while.

Thanks for the detailed answer.
I only started the game at release & playing on domination, had never seen as high shield items so i guess i see how it's manageable, however the guide saying 10K was easy to reach is quite optimistic, based on this i'll be happy if i reach 7K without gimping my build. Definately did give me some ideas to get to 5K+ without downsides, thanks!

It's "easy" from the perspective of the build designer. Since you're reading a summoner guide, the person is most likely deep into the lategame(summoner builds shine lategame), so getting great ES items isn't that hard for them. Even then, his statement was misleading. It's "easier" to get 10k ES than 10k life, because getting 10k life is a pipe dream.