Do Tribunes join army without experience?

[CaesarAugustus]

Hey I have wondered how the class structure of the military works. I know Legionaires are the basic ranks made up of citizens. A veteran Legionaire would be promoted to Centurion. Most Centurions were lower class rarely make it to Tribune unless they become Senior Centurion.

My question is how do High class Senior officers like Tribune to Legatus join in the first place? Do they have to serve with other lower class Legionaries and rise up the ranks to Centurion and then Tribune facing multiple combat actions? Or do they just get elected and join the officer corp without any experience? Or do they have a ROTC thing where they train at home in an officer school with upper classmen until they are ready to command?

Like Caesar for instance. He was banished from Rome by Sulla and had to join the army. Did he join the army as a Legionaire and rise up the ranks?(Legionaire's couldn't marry, but Caesar was married....how did he join?) Or does he join automatically as Centurion or Tribune officer without any experience in war?

More or less as you have said, upper classes entered directly with higher ranks. But, at the same time, we have also news about common people who were made directly centurion, thanks to acquaintances with high spheres.
But it is also true that the sons of the aristocracy often accompanied the legions, sometimes without an official role.