could you guys help me? I have been playing the game and I noticed some things

I often when i need expand my territory i do:: 1) expand departments 2) create a city 3) create an outpost

However, playing in the campaign, I noticed that the computer greatly expands the territory without using these options...

What am I doing wrong?

It is not that you are doing it wrong.  It is that there are multiple ways to do it.

For example, you can grab multiple districts with outposts before a city is ever created.

it is all about how you prioritize your actions.

i do this.....

but I noticed that in the mid game, some cities located relatively far away, have a disproportionate territorial extension! Even though there is no outpost and no other city in the region

If there's an unconnected city, it's usually from assimilation of an independent person (barbarians). You can invest money or influence in them and when you get to 100, you can assimilate them which means it becomes your city.