The most common hand is ╭∩╮. The two curves come from the Box Drawing block, which was made for drawing frames in old text screens. The finger in the middle is ∩, the math sign for intersection.
╭U+256DBox drawing light arc down and right
∩U+2229Intersection, the finger
╮U+256EBox drawing light arc down and left
The shorter 凸 is a Chinese character. It means "convex", something that bulges out, and its shape does the rest. Its opposite is 凹, "concave". The simplest version is t(-_-t), where each letter t is a hand with one finger up.
Text hand or emoji?
There is also an emoji, 🖕. It was added to Unicode in 2014 with the long name "reversed hand with middle finger extended". Some apps and games hide or blur that emoji. Moderators treat a text hand the same way, because it means the same thing. The gesture itself is very old. It has meant the same thing since ancient Greece, as the article on the finger explains.
When is it okay to send?
Between close friends as a joke, most people take it fine. At work, at school or in a game with strangers, it can hurt feelings and get you muted or reported. For a kinder way to show you are annoyed, try an angry face or a shrug.