It varies by army, so this won’t be exact.

A platoon has around 30 people in it. It has around 20 privates, about 7 non commissioned officers and one commissioned officer. Say 3 lance(junior) corporals, 3 corporals and a sergeant. In overall command is a commissioned officer.

When you join the army you typically go in as non commissioned as a private or commissioned as a leuitenant.

Officers are trained to lead people from the start. The lieutenant may have 1-2 years experience and the sergeant may have 16 years experience.

The next level up is a captain who commands say 3-5 platoons. At the start of saving private Ryan, Tom Hanks character was a captain in command of around 100 soldiers.

I think around 5 companies make a battalion. And that’s just soldiers.

You can also make a unit out of soldiers, engineers, artillery, transport and some other specialists like medical and cooks.

I believe a brigadier general would command a unit around this size.

To get there I would expect he worked his way up the rank, working at each level for around 2-3 years.

He would have been in the army around 15-20 years, but some people could get promoted faster.