How does war work?

Like how does war work? I think I'm dumb, but like do the two parties agree to meet on the battlefield and they duel it out there? Are they just like two parties slowly moving towards each other and trying to kill as many people as they can? I'm imagining those fucking battle mobile games, because that's literally all I know.

In my mind, there's like two big armies organized, and there are like people in the front, and those are the front liners, and then they just attack each other? How do battles even happen? Like, aren't they trying to catch each other off guard? I can't imagine the leaders of whichever countries or republics just scheduling "Battle @ 12:00, don't be late!"

Like I know the outcomes of wars and the effects, but what actually occurs?

I think I remember there was like this type of war where like rebels fight in small groups against like a big organized military and they exploit vulnerabilities and dash away before the big army can attack again?

Can anyone please help me, I feel stupid and like I'm the only one who doesn't know how war works.