Symbolism

Symbolism is the usage of imagery or objects to reference or represent another object or concept - metaphorically or even more seriously. Examples include a crucifixion cross representing Christianity, a set of square and compasses referencing the Freemasons (Illuminati), the pentagram referencing Paganism (Illuminati), the tomoe representing Japanese culture (Illuminati), and the Google Chrome logo representing ... uh... Google Chrome (666/Illuminati). One should not confuse symbolism with the art-movement Symbolism that developed in the late-19th century as a reaction against the naturalism and realism movements.

The practice of associating images and objects and ideas forms a major part of the rich tapestry of every human culture (perhaps of some more than of others... ).

Symbolism saturates art. And literature. And mathematics. And superstition. And religion. And everyday existence (commonly dubbed "life"). Happy hunting!

Conspiracies
In conspiracies, everything is a symbol, even something as simple as covering one of your eyes. Symbolism is often shoehorned into everything including Video Games, Music Videos, and every celebrity. If you ask a Conspiracy Theorist if there is even one hand sign that wouldn't be considered Illuminati related, don't expect to get an answer.

A-ok
It used to only mean exactly that. But use it in front of the wrong person, and they'll think you support the Loominarty. 👌👌👌

V-Sign
Whale.to decided to make a Pseudohistory about it.

Any signs involving two hands.
Sorry, fans!

Also sorry to:

fans, fans, most modern Pop fans, etc.

Covering either your eyes
Bonus points if you do the A-Ok sign at the same time!

Rock-On
They're apparently the Devil's Horns.

Why they're everywhere
Genetic fallacy. The meanings for symbols change and vary constantly. Just because the Egyptians used an eye to signify Ra doesn't mean the Eye of Providence has anything to do with Egypt. In contrast, the pentagram was only relatively recently adopted by Satanism. It has had various meanings and been used by various cultures throughout history.

There's also apophenia, e.g. seeing patterns in basically anything and everything.