Hello there. Not discouraging anyone from providing additional information (More can always be said about this topic), You should definitely check out these older answers :

How did large armies know where they would find one another in medieval times? by u/Gadarn

How do armies agree where and when to fight? Specially during the Crusades or Alexander's time? by [deleted]

Imagine you are invading France (just a example), due to the fact that France isn’t your land you are going to need a army, and since France has a big army you are going to need a bigger army. Since you have a bigger army you have to stick to the roads, so that your army is organized and can’t be ambushed. If your huge army is marching and pillaging around France, people will notice and the French king will march towards you with a army. You find out about this (cause it’s another big army marching in France) so you decide to fortify yourself. Sending out to attack supply carts and commit raids using some troops doesn’t do much if they are protected, you are basically wasting your own soldiers. The French can’t assault you cause then they lose soldiers attacking a fortified enemy, and they need those men to fight you. Besieging isn’t fun for either side, the invaders have to sit around and be spread thin and might get raided, while the French have to starve. Besides the French king isn’t just gonna ignore the fact you pillaged his farms, which means he gets less money. So you meet up, cause it suits both interests to fight the battle. Armies cost a lot to maintain, so a quicker campaign is good. So why don’t you just crush the enemy army sooner then later? You obviously can spot each other cause it’s thousands of soldiers (you can’t hide that, especially when your soldiers have armor that is noticeable and your banners are always colorful). You can’t outmaneuver the enemy cause it takes days for a big army to move anywhere, and if you outmaneuver them, they will noticed and just outmaneuver you. Besides if you are invading you need food, you need those supply carts, and if you run outta food it’s game over for you. If you just sit in your fort then you starve and die. The actual battle usually begins with the army preparing and getting in positions, once one army does this the other one does. It’s not good to fight in a camp for either side, so a field is 10x better. You have sergeants leading each unit and giving orders, and you have a horn or other vocal cues to begin your attack, once you attack they attack.

Simplified version but a insight into the current thought process for most pitched battles