How did a formed infantry unit get past a friendly unit already standing in its way?

[u/chad_kun5145]

I'm making a real time battle game set in Warring States China (260 - 220 BC). Each soldier is simulated individually, and units of about 100 men fight in formation. The movement is what I keep getting wrong, so I'd like to know how it actually worked, not how other games fake it.

What I'm stuck on:

1. Moving as one piece. When a unit of ~100 marched across a field, did it keep its exact ranks and files the whole way? Or did it move loosely and re-dress when it stopped? How much did the lines wobble in practice?
2. Turning. When a block had to face a new direction, did it wheel, with one end pivoting and the outer files walking faster? Or did every man turn on the spot, so the back rank became the front? Which was normal, and when would you pick one over the other?
3. Getting past friendly units. This is the big one. Say a rear unit has to move forward, and a friendly unit is already formed in front of it. What happened? Did the front unit open its files to let them through? Did the rear unit march around? Were there deliberate gaps between units for exactly this, like the intervals people describe for Roman maniples? Or did the two units swap places some other way?
4. Gaps in the line. In a battle line made of many units, did units stand shoulder to shoulder as one continuous front, or with gaps between them? If there were gaps, how wide, and why didn't the enemy just pour into them?
5. Replacing a tired front unit. During a long fight, how did a fresh unit take over from one already in contact? Did the tired unit pull back through the fresh one, or did the fresh one push forward through it?
6. The charge. In the last stretch before contact, did men run? Did they keep their ranks at a run, or did the formation loosen on purpose?

Warring States China is where I need answers most. But if the good evidence is Greek, Roman, or early modern drill, I'll take that, and I'd be grateful for an explanation of what probably carried over and what didn't.

Book or source names are hugely appreciated. I'd rather read the real thing than guess.

[u/Gasmask134]

I can't really give any answers of the time periods you are asking about, but information from the 19th century, manly the American Civil War, might still be of some use as a general idea (after all they were also attempting to solve the subtlely difficult problem of marching a lot of people in formations)

1. Units marched in the formation prescribed by the commanding officer and did their best to maintain that exact formation as they marched. However, it was common for a unit to slowly start moving towards whatever was decided to be the guide (ie the unit colors or the left/right most company) or sometimes one part of the formation may wind up further ahead than others due to a delay cause by not hearing and processing an order to stop until later or terrain. When this happened having a unit stop to reorganize themselves before starting again wasn't unheard of. Earl J. Hess' book on ACW infantry tactics has an anecdote about an officer having to rush ahead to the color guard to get the entire unit to stop moving forward as they couldn't hear him, followed by having to do this a second time to guide the color guard towards the rear with the rest of the unit following.
2. This depends, a unit could wheel, but this can be a difficult thing to pull off while engaged. Turning on the spot, or marching by flank was an easy way to turn a line into a marching column which could then be redeployed into a line upon arrival at a position
3. This one is a bit of a mystery, Hess talks about units sometimes stopping and tightening formation as retreating friendly troops approached an advancing unit, failure to do so often led to the advancing unit becoming separated as companies get dragged away by the retreating or retiring troops. There was improvised methods to get around this but it often required a bit more advanced warning to the commanding officer that someone was to pass through their line, an example Hess gives is of an officer having regiments at the front of the line wheel into a column to form a gap to allow the second line to pass through, but often officers were light on details of how exactly this worked.
4. This one is also a it depends sort of situation and I'm not sure how helpful an ACW battle can be for a warring states period China, but usually a unit is occupying important ground and often has a reserve line behind them that can be rushed in to refuse a flanking attack. A gap was ideally big enough for a unit to be able to freely maneuver around terrain, but close enough that two units can support each other if an enemy attempts to flank one of them.
5. This is a thing I am not as familiar with, but ideally you want the unit to maintain some semblance of formation as it reaches the last stretch, which usually means holding off on running until the last moment, obviously men will have a hard time maintaining a formation at a run with bayonets before an enemy, but if you keep them marching in good order up to that point you can mitigate how spread out they become for maximum impact.

edit: most of this is about more regimental and brigade level stuff than 100 men as you ask. 100 men is roughly a single company and is usually a small part of a larger unit that actually conducted manuvers