In short: messengers.

This question is very open-ended, so I’m looking at mostly warfare in Europe and North America. I’ll try to specify particulars where I can.

Often before a battle, corps or division commanders would meet and discuss general plans for a battle. This assumes, of course, they knew the enemy was nearby, and generally what ground they would choose to fight over, often when one side was in a defensive position already and expected the other to attack. During meeting engagements, such as the Battle of Gettysburg (American Civil War, 1863) this wasn’t really an option, as both sides were feeding troops into a battle as they arrived—the ground was chosen by where the armies met, and neither at first expected a major battle.

Of course, enemies are notorious for not going along with battle plans (no plan survives contact with the enemy), so messengers would be the most common way I know of to pass orders along. Flags were a major part of this—they marked locations of units for their commanders, and were a reference point for messengers going to find the subordinate. Sometimes the general might go in person, or corps commanders would meet at the general’s headquarters to receive orders. Again referencing Gettysburg, General Meade (commander of the Union Army of the Potomac) found out that General Sickles (commander of III Corps) has moved his corps forward from its assigned position—and was under attack—when Sickles failed to show up to a meeting of his corps commanders.

This system wasn’t perfect. Messengers could be lost or killed, or the messages intercepted by the enemy. This is not an example of a message in battle, but during the Maryland campaign in 1862 a copy of General Lee’s orders splitting up his forces was found by Union forces, which could have proved disastrous if a less cautions commander than General McClellan has been in command.

The flip side of this is, if a general’s subordinates were good commanders, they could be given general instructions and react to the situation as they saw fit.

Some variations: prearranged signals were sometimes used. I am not as familiar with the Thirty Years War (and it’s technically outside of your time frame, although still relevant), but in some of my readings I have seen references to the artillery firing a volley all at once being a signal to begin a general advance. The Union Army experimented with semaphores (flags which could be waved in different patterns to pass messages), with some success. Telegraphs were also in use in the American Civil War, though these would not be used for battlefield communications, for obvious reasons, more for strategic coordination of armies.

As for how they’d know where the enemy was—that was a difficult prospect. Cavalry were traditionally used in a scouting role in a strategic sense. Line units would often operate with skirmishers out ahead in open order to feel our enemy positions. And the sound of gunfire was always a good indication. But battlefields were chaotic, noisy, confused things, and it was near impossible for anyone to have a completely clear picture of the disposition of their own forces, much less the enemy.

Hopefully this has shed some light on the topic for you, and wasn’t too confused abs rambling.