How did armies clash at each other? | AskHistorians Archive Observer

That's quite a list of questions you've got there, and I will give you what answers I can. Most of what I tell you will be sourced from Maurice's Strategikon, a Byzantine manual dating to the 6th century, as well as being backed up from my readings of Clausewitz's On War, which is comparatively modern.

First part, how the battle happens: The idea of the battle where two big armies face off dramatically is quite rare in history, as such a battle is risky for both sides, and very costly even to the winner. With that in mind, you can see that nobody is in a particular hurry to engage the enemy without first ensuring that everything is prepared.

There isn't a ritual, or a set of unspoken rules for the battle, so much as both sides typically hurry to take up a good position (commonly a slightly higher ground). Typically, when this sort of battle happens, there is one side that actually wanted the pitch battle, and other other side who does not (usually the smaller of the two armies). As such, the aggressors would be inclined to move up and engage the enemy, whereas the defender would await at a more advantageous location. Source: Paraphrased and summarized from On War.

Now, with that in mind, we move on to the actual clash itself. This doesn't always involve the Hollywood style hordes of men smashing into each other at full speed. Human beings are typically not entirely willing to get themselves killed, as such, the approach is typically very measured and steady, in order to allow the troops to keep in tight formation and protect each with shields, and to prevent individuals from being separated and surrounded.
Source: Strategikon, paraphrased and summarized.

There are variations from the above description, depending on the particular situations and time period, so if you wish for more details, narrow down your question a little.