Cluster Jewels (I'm lost asf)

So I keep seeing cluster jewels in builds and how they are important but like how tf do they actually work? Like I don't really understand if I need to save points to use them? or if they are an in-game drop that I can add into my skill tree?

They drop from Delirium and you can place them in the sockets at the edges of the skill tree. Largr cluster jewel can have 2 medium ones socketed in it (or normal jewels) and a medium one can have 1 small one.
And yes, you need skill points for them.

Think of them like customisable passive tree extenders.

You can place any cluster jewel in any of the outside ring jewel sockets. Large CJs have 2 additional jewel sockets, and medium CJs have 1 additional jewel socket. You can place any jewel except other large CJs inside the large's sockets and anything except large or medium CJs inside medium's sockets.

This means a full set includes: 1 large, 2 medium, and 2 small. Furthermore, they can be crafted in several ways to either minimize points spent, maximize the value of each small notable, and provide several notables not obtained elsewhere on the tree. Furthermore, because these are not allocated on the tree itself but added to it, you can have multiples of the same notable passive, and they stack.

Cluster Jewels Usage
To use a Cluster Jewel, open your skill tree, find an 'Empty Cluster Socket' node (located only on the outer ring of the skill constallation) and ensure you have allocated it by investing a skill point to it. Then, open your inventory while the passives menu is open and place it into the socket. This will add its corrisponding skills to your tree as long as it remains in place. Clusters can be removed, but all allocated skills will be removed with it (if you have any allocated skills there, all of them will be refunded for 1 Refund Point).

Note, you can dasiy chain clusters, as Large Clusters come with 2 Medium Sockets, and Mediums come with 1 Small Socket. In the extreme cases, this means a single Large Cluster Socket can cost up to 36 Skill Points at its highest value if you invest fully into a max Size Large, Two Mediums and two Smalls (Mimimum passive clusters are often valued to reduce this down, and not all cluster builds use a full set, sometimes they just use a Medium or whatever).

Cluster Jewel Properites
Cluster Jewels will have an implicit number of nodes, and a base effect for those skills. Large, Medium and Smalls have differing pools of modifiers, from Fire Resistance, to Armor, to Minion Damage to Reservation Efficency. These cannot be changed, and finding the right jewel as a base to work from is the first problem to deal with (ensuring a proper iLevel, skill count and base ability).

Once a base is found, they can be modified like any item via Transmute / Alchemy / Chaos. The properites modify either what the passives provided offer (like an extra +2 strnegth) or offer Notable passives that are often the draw for some builds. These notable passives are tied to the bonuses implicit in the Cluster. Thus a Cluster with a bonus to ES will have ES related Notables. The pools are fairly shallow, so it is possible to brute force chaos re-roll into a viable product if you really need to. There are not quality modifiers for Cluster jewels.

Where to find Cluster Jewels
As everything in Path, buying will be the simplest route. But if you've a mind to make your own, Clusters come from the Delerium mechanic, and there are several Atlas Passives that can greatly enhance your ability to find Delerium events (That which you seek) or Cluster Jewels (like Paranoid Fixation). However, you can also find them from Heists in Jewelry chests regularly, but this should be considered a supplmenet, as it's not reliable enough to find enough clusters to filter for the one you're looking for.

Do you Need Cluster Jewels?
You can fully do any content at any level without cluster jewels. However, if you are following a build that uses them, I'd generally suggest trying to follow suit. Read their notes to see if any mention of the importance they put on it. Some builds can live without their intended clusters, but it can reduce your total power, something from just a 10 to 15% less damage compared to putting those skill points elsewhere, to a crippling loss of power if the build is really reliant on those clusters. Without seeing the build it's impossible to say if it's using the clusters to eek out a bit of extra damage, or if it's totally relying on a part of those clusters for its end game. Though my generally suggesting to very new players is to ignore clusters, as they are a complicated element (And god help you if you need a Legion Jewel).

Generally speaking, if the area of the tree has what you want (be it life, damage, etc.) you are better off spending your points on your tree. Cluster jewels are there for if that part of the tree is lacking in what you want (i.e. if you are a dot build, there is very little dot specific damage in most parts of the tree). There are exceptions to this of course but for the most part, cluster jewels are mostly good if you have points left with nothing good to take efficiently.