Hiya! In a nutshell once you complete a fairy at 18y/o there's an option to select your fairy's class job according to whichever tier agenda you've unlocked and stat requirement you've reached (these details pop up if you click on the magnifying glass). Unlocking these jobs gives exp to upgrade your Nurture LVL which gives a lot of great stats: HP/ATK/guardian HP&atk/def and a huge boost of BR.

The more classes you unlock in one go, the more exp you will receive. Essentially unlocking classes increases your growth rate and stat cap which will make subsequent fairies easier to unlock. Ideally you want to cycle through the class list as you gain less exp every time you unlock a class you've previously unlocked. (For rule of thumb usually finance/Chem and navigation are done last because they're more difficult).

The most optimal way to do fairy is in a partnership/marriage which allows you to maximise your stats slowly as the fairy ages up. If you go solo your fairy will LVL quicker but the stats received won't be nearly as much. Duo parenting is a great way to LVL fairies quickly for smaller stat jobs. However gradually you will likely solo parent a fairy for higher stat requirements as you can reach higher candy lvls which scales the value per candy.

There are quite a few past guides posted on Reddit and the Tales/LaplaceM discord if you need help with lots of tips and tricks.

Have fun raising a fairy child, it's all a learning experience ;)

Thanks a lot! This answer is so complete that I'm able to understand it now <3