1: Why does a mage need to level up their magic? So that they can attack with magic.
2: And why does a mage need defense? So that while they are attacking with magic, their army doesn't die quickly. In other words, by increasing the mage's defense, your army, which is weak in attack, can withstand blows well.
3: I didn't understand the question about which unit to apply Necromancy to. You're probably talking about a racial ability. If so, with this ability, you can resurrect your killed units and, conversely, attack the enemy.
4: Why does a hero attack with their hand? Most often, this is used when there is no magic or active abilities, or to finish off a weak unit that is hindering, but you don't want to waste magic on it.
5: How do ghosts work? They work the same way as Necromancy. They raise fallen warriors and damage enemies.
6: Restoration is magic that heals the wounds of your units, that is, you cast it on those who are wounded or on those who are about to be wounded.
7: There are many reasons why they break units into single units: For example, when you attack an enemy, they attack back once per turn (except for certain units and if the enemy doesn't have a skill for additional counterattacks). So, you attack the enemy with a single unit, it kills it, and then you finish it off with your other, stronger soldiers. This is convenient against a strong enemy.
Another reason to break units into single units is that it can block a passage, thereby delaying the enemy's advance towards your army for some time.
Please, ask questions again, but list them by number so that we can discuss them if necessary