There is no attack roll in 5e that "both" sides roll. Either attacker rolls against Armor class, or the defender rolls "saving throw" against that ability's difficulty check "DC"

So both fire bolt and a crossbow bolt is going against armor class. Hence AC is very, maybe too, important in 5e, hence increasing it is hard.

Attack roll is; if you are using your class ability, modified by class stat bonus. If it's 20 it's critical, spells can crit this way if they have attack rolls.
DC is again determined by stats. If you are wizard casting a wizard spell INT helps. But if you are using a wand /scroll of a different class on PAPER it does not help, i don't know the game yet.
Defender's saving throw is modified by corresponding stat. There is no special crit fail or crit success saving throw on 5e. You save or fail. (That's on pathfinder 2.0 laugh Yay! )

Special situations can give you advantage or disadvantage, which means roll 2 d20 dice, take better result, or worse result accordingly.

He who fights with monsters might take care lest he thereby become a monster. And when you gaze long into an abyss the abyss also gazes into you.

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