The Hero's journey is very individualistic. I want to know how writers build the book's structure for multiple characters. Do they make one for each character of for a whole? If it is for a whole, how they do it?
You cannot treat the group as a single unit.
There is no such thing as "the gang is the protagonist". You can have a group (a gang, a party) with individuals, each with their approach to the story and their story arc. Some will be more developed than others.
Every character has an Arc of transformation. They start in a specific situation and with a specific knowledge, they go through events, and they end up with a different situation or knowledge. Every Arc has its own turning points: you can do them individually for each arc, or you can make them coincide. Basically, I think you can draw each character arc exactly like the main character's one.
What is the difference, then?
In a multi-thread or multi-character story, it's not just one hero who carries on the values of the story. Every other characters share part of this function. They have a different way to approach and answer the main question of the story.
Usually they relate to the main character somehow, so they propose an alternative approach or attitude towards the main problem or the main theme. For instance, one protagonist or main character carries the main question of the story, and each character poses a different answer. In other stories they are more independent, and the story is a collection of individuals stories under a general umbrella which is the main theme.
To the extent you plan out arcs at all (many of us do not, and Stephen King does not), make a plan for each character present for a significant part of the story.
For example, in 007 movies, Q is the gadget guy that shows Bond all the cool things about his car, his wristwatch, his pocket devices, etc, that will of course be the deus ex machina that lets Bond escape some otherwise impossible situation. (How Q became so omniscient to give Bond the perfect device he would soon need, different in every film, is not explained.)
So while Q is important, he needs no character arc. He is a prop, like many characters, there to accomplish the exposition of explaining Bond's devices and how Bond came to own them. Q appears in one or two scenes and that is it.
The more often the character is making decisions that affect the story, the more of a character arc they need. (007 is a bad example for this, he virtually never changes at all.)
Or if you don't like the notion of character 'arcs', you might consider just character transformation within the story. The idea that the adventure teaches them, changes their attitude or philosophy about something, affects them in some way by the end of the story. Matures them, for example.
Which is why some of us discovery writers do not PLAN arcs, we try to let our characters be as much like real people as we can imagine and make decisions, have emotions and slowly change who they are throughout the story (much as people change in real life). Thus by the end of the story the 'arc' has taken care of itself, my characters at the end of the story are not who they were when it began. But I did not say "At 70% through the story I must make Karen realize how cruel she has been to her sister," or anything like that.
Such inflection points IMO are the culmination of escalations that people naturally engage in, so Karen's cruelty her sister's responses will escalate until something breaks her sister and Karen finally feels guilt for it. An arc with turning points will exist, to be sure, but it does not have to be planned.