The restances are the same as yours afaik. So it a value calculated from the attributes. The hero also has this and has saves againts magic or elements. You can see your stats on this in your character screen. They increase if you increase the related attributes. This is the same for monsters

Some monsters are more resistant to elemental magic others to divination magic. Some are horrible to kill in melee. You can go for ranged weapons but then have to pay for arrows and you can miss with ranged weapons, too. In addition arrows dont replenish like mana just for free.

yep they have resistances so chance they don't effect them at all. Easiest way is just savescum. quicksave your game, cast on them. if it hits great, if not reload and try again. Just like most things in the game you can save almost anytime and reload. Save before casting. Save opening chests, save before a fight, save when you want.

So yes, you might cast fire darts on a minotaur and it actually affects them 50% of the time. Or you can savescum and it will affect them 100% of the time.