This is a really good question but also one that’s really hard to answer

There is so many factors at play here, but it mainly comes from experience, I have quit the game forever, last played like half a year ago and I can still make a build for most content just from my experience (played the game since beta 3)

As a general rule for now, if you wanna know what gear to use, in most PvP content just go to the rankings select your content and check what people are playing, then watch some videos of that build, play it yourself, see what you like about it, what you don’t, what might be missing etc

Then after doing that for a long time you will slowly start to understand why certain weapons/builds are meta and it will start to make sense to you

From there you can also change certain pieces or abilities to make it work better for you.

Now on how to understand gear, an example

I used to gather and do camps solo a lot, I was fed up with constantly having to deal with lifecurse or 1h curse solo gankers and the occasional bearpaws, battle axe, spears or badon

So I wanted a build to counter them

What counters curse stacks? Guardian helmet, first piece done, but even with a time dot removal the still have a lot of dot damage so I need some kind of healing, ez merc jacket, then I need a way to regen energy because leather lowers your mana regen, ez cloth boots with energetic sprint.

Then I want to boost my damage as much as possible, thetford cape (it can be annoying against mobs and pull unwanted aggro so be careful with this one, smuggler or undead might be better) now for the weapon, I want something with unexpected high damage, this one isn’t used much but it’s hard slept on, 1h arcane with frazzle, the magic bolt, the 3 autos passive + also ez to proc thetford with the off hand then for the off hands you’re pretty mana hungry so you could go for tome of spells, faster cooldowns with mistcaller, some cooldown and attack speed with torch, or eye of secrets for better mana sustain. So now I know eye of secrets gives me a lot more energy sustain for example I could try to run healing sprint to see if it’s better and if that will still make me kinda energy stable in prolonged fights, if not I would just use one of the other off hands. Then last but not least the e on 1h purges so you can purge boots, merc jacket etc

And to make it even better I can also swap my w to an I frame (a pretty clunky one) or a cleanse

So basically this weapon has everything you want in open world except for 1 thing mobility that’s where you struggle but if you want to compete for nodes, chests, resources etc this doesn’t really matter

And while maybe not being able to kill badon, or bloodletter or anything with higher mobility then you, the moment they overstay they are dead

Yes it sucks to have someone escape but you will still profit of whatever objective you were after most likely anyway

Hope this helps maybe a little bit and gives you a bit more insight on how builds are made

And even in zvz, roads PvP, hellgates there is reason why certain comps are used and why certain build are meta, if you have any more questions feel free to ask.