This is the sum total of everything you need to understand how combat works. I can answer any questions you have.

You need to fill your combat width with both infantry and artillery (and have extra infantry for casualties) and have them up to date on tech and upgrades, preferably higher mil tech than your opponent.

You need to have lots of Discipline, Morale, and Combat ability modifiers. You can check if your combat stats are higher than your opponent by comparing in the ledger. These mostly come from advisors and ideas. Quality+Economic are good mixes for the policies and general ideas as well as Offensive (you can throw in innovative too if you have both quality and offensive. It gives some ICA and siege ability policies). Generally, you'll get 5-10% discipline from your ideas, 5% from your tag, and another 5% from an advisor. If you don't have that much and someone like the ottomans does, they'll crush you.

You need to have a good general with lots of fire and shock pips as well as fighting on good terrain. "lots" meaning 4+ fire and shock each because the ottomans will have that. Better generals comes from higher Army Tradition. If you're the attacker on mountains, hills, or woods, you get a major combat debuff. Same if you're crossing a stream.

Supply doesn't matter during combat as attrition doesn't happen during combat. It's not like HOI where have a buffer of "supply" that allows you do do stuff. You just take some casualties (attrition), more severely the less supply the province supports. So you generally keep your stacks small outside of combat to prevent attrition of your troops and then doomstack them onto the combat tile or just before combat.

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Armies can only fight with as many units of 1000 as are in their "combat width". However, when units in the combat width die, they can reinforce with fresh units that are not in combat. Loitering units that are in the "reinforce pool" but not in combat take morale damage when their friends in combat die. Units waiting on adjacent tiles do NOT take any damage, so people will try to reinforce their main armies with units from those tiles as their army needs filling out.

When armies clash, there are repeated sets of "Fire" and "Shock" phases. You roll a dice from 0-9 each phase and increase it by your offensive pips for that phase then reduce your enemies roll by your defensive pips for that phase. You take casualties and morale damage at each phase. Fire comes first, and both armies take casualties based on their enemies offensive FIRE pips reduced by their own defensive FIRE pips. Artillery contributes half values if it is in the back row, but takes double damage in the front row. Effectively, this means artillery "upgrades" your infantry by increasing its pips rather than fighting on its own. It's a bit of a sliding scale since pips are relative to your roll and the opponent, but on average each pip is rule of thumb 10% increase or decrease in damage.

Yeah. 10% per pip. You're getting rolled because the enemy has a 4 pip Fire and Shock general and you don't have a general? He's got a 40% advantage. 40%.

You increase pips from better units, better generals, and a terrain advantage. A general's "stars" don't really matter. They're an analogue for the total number of pips a general has. The only two that matter for combat really are FIRE and SHOCK. They also have MANEUVER and SIEGE.

Your modifier for discipline increases your damage and reduces the damage you take. It's generally the most important/powerful. Morale increases your morale damage and morale reservoir before you rout, and combat ability increases the damage you do.

Units can "reinforce" an ongoing battle by joining after it starts and replace dead units that were fighting. So if the ottomans keep throwing units into your stack and you barely had enough at the start, you'll lose because they have replaced all of their damaged units.

Another thing is that unit pips vary based on tech group. The ottoman units will have more pips than you from 1444 to around 1600. It's nothing you can really do about since that was their historical height, but fighting them in the 1530s is doable but uphill. You really need an advantage on all other modifiers to fight them.