I have several questions in regards to the historical accuracy of Roman units in Rome 2. Where do legionaries fit in, etc?

So I know that the Roman military originally used a hoplite heavy military but soon switched to the maniple system and after that moved on to cohorts. The game doesn t allow for the first phase of hoplites but you start off with the maniple system with all the relevant military units.

The confusion for me starts at legionaries. They aren t cohorts but they aren t a part of the maniple system either, so what are they? What also are the first cohort and eagle cohort? What historic significance do these 3 units hold?