Professional Army

I need some clarification on professional soldiers here because my brain isn't processing this too well. It says "The professional army does not have age groups, it has no sick or wounded and when soldiers age, they are automatically replaced by new recruits. It does however have losses during war and in fact, losses are always assumed in the professional army if the country has such an army." If they are automatically replaced by new recruits does it mean if they die in war they're replaced or do they just flat out die and you can never get them back unless you purchase more? Also can they train your regular soldiers to become professional? I need some answers before I think about spending my precious gold haha.

The documentation basically states that if you buy professional soldiers, the only way their number will decline is by their death in combat.

For the sake of 'realism' lets assume that whenever one of your professional soldiers reaches the age limit or gets sick/wounded, he/she instantly gets replaced, free of charge.
So, for all intents and purposes they do not age or get sick/wounded. They will be there until you get them killed.

Thank you so much for answering my questions^^

" They will be there until you get them killed. "

Sounds so... gloomy, until YOU get them killed.

Ikr ;-;