In English a lot of people informally call their friends these terms in conversations.
Like "Dude, what are you doing?"
"Bro, what's up!"
My question is how do you informally speak to friends in Japanese?
Like, how do close friends greet each other? Is there a word like dude or bro, used to informally refer to other people?
I don't think there is an equivalent concept in Japanese because the most informal way to address someone is by using his/her name without any suffix. It feels pretty normal to call people by their name in English but Japan is a highly hierarchical society so not using those suffixes (called 呼【よ】び捨【す】て) can be really rude if you are not close to the person you talk to.
There are actually quite a number of informal pronouns in Japanese, but they would not translate exactly 1-to-1 to those words you have in English (Well it's a different language), especially when these are casualish terms, they don't always have a perfect translation as they are often derived from very localized contexts. I am assuming that part of your question is related second person pronouns, as it seems like that all your quotes were for second person references. I can list some of those here, but beware, some can be rather rude considering the context, and most of these are definitely not suitable for talking to superiors.
Literally: the one in front (前) of you (+ honorific お). Although it looks like it is honorific, in reality this is a rather informal pronoun to use.
According to this source, "... often used by between male friends... and isn’t rude (if you’re friends), however can be used in a rude way as well."
Additionally, according to this source, "used in very informal situations or toward people of lower status. This word feels very “blunt” and can easily come off as rude."
Note that this is rather different to "あなた" connotationally, even though they sound similar. This word can have a sense of looking down on the person depending on the situation, according to this source.
Literally: in front (手) of hand (前), i.e. the one in front of your hand (you). Slightly derogatory, can be used for discontent, etc. Can be used for a joking reference if you are really close (like die-hard friends) to the person, or used by a superior to inferiors who are really close in a very informal situation (maybe something like jokingly referencing to an inferior when they are drinking at a bar?)
According to this source, "this is likely only meant to be rude, and is ... derogatory ..., meant to show disdain and often heard in anime/manga."
Additionally, according to this source, like other derogatory pronouns on this list, "[they] all indicate anger and/or disapproval of whoever you’re talking to. ... don’t use them unless you’re trying to pick a fight."
Literally: The noble (貴) (+ the noble honorific suffix 様), often used in a sarcastic tone, or looking down at the person referred to.
Archaic word seen mostly only in manga and anime according to this source.
According to this source, another derogatory pronoun, although rather archaic.
As for the other part of the sentences you have referred to, about "what's up", etc. and things like that, there are a lot of different ways to 挨拶{あいさつ}する (whats the word in English?) with close friends in Japanese. I can list some here as well. Note that in Japanese, when the context is clear, subjects (or other parts of the sentences which have become clear to both parties in the conversations) can often be omitted, as such, in casual conversation, it is likely that the pronouns (dude, man, etc.) will be dropped, leaving simply things like "S'up".
From this source
From this source
According to source: "Originally an extremely formal word used in the military, this word is still commonly used by martial arts practitioners. Nowadays, it’s a slangy way to say hello among young people..."
According to source: "... it sounds less stiff. You use it when you meet and greet friends. And this is a tad more conservative than おっす"
According to source: "This question can be used at social gatherings with friends and is a safe phrase to ask people how they are doing. It can mean “how have you been,” “what’s new,” or “what’s happening.”"
In general, there can be many variants to the same linguistic variable that perform similar functions as in above. This list is in no way exhaustive and only aims to list a few of them as examples. I hope this helps (:.
P.S. I guess the best way to learn this is actually get some Japanese friends and hear out what they usually use in normal conversations (although I am guessing that these informal terms will really highly depend on where they come from, and the linguistic backgrounds of them)
