I'm not a DnD player and only entered the franchise with BG3, but some additional infos, I hope they are correct. Astarion is of the Rogue class which has proficiency mostly with "simple weapons", like light crossbows and short bows, but also f.e. with longswords (for Strength based Rogues, hmmm) or short swords or hand crossbows. A longbow is not a simple weapon but a martial weapon. Astarion is however by race an high elf who have proficiency with some martial weapons, like longswords and longbows.
So it would make no difference wether Astarion had a light crossbow or a longbow equipped, both are 1d8 damage (1d8 die translates in an average damage of 4.5) and both take their damage modifier from Dexterity (the "damage" stat of Astarion, till lvl 4 he has 17 Dex, which add +3 to the weapon damage of ranged weapons and "finesse" melee weapons). A longsword is not a finesse weapon but based on Strength. It would be a bad idea to give Astarion a longsword although he has proficiency because he has not the Strength for a good added damage. He has just 8 Strength, that would "add" a -1 to the damage (at least from the tooltip, never tried). So with a longsword he would do 1d8 - 1 (= 0 to 8 damage = 4 damage on average), with a short sword (finesse weapon) he would do 1d6 + 3 (4 to 9 damage, or 3,5 + 3 = 6.5 on average). A 1d8 finesse weapon like a rapier (for which Astarion has proficiency too) would do 4,5 + 3 = 7,5 damage on average.
However, because the short sword also is a "light" weapon, unlike the rapier, a second weapon could be used in the second hand, using a bonus action for attacking, which adds damage and is the better solution as a single rapier. Later with the Dual Wielding feat weapons without the "light" feature could be dual wielded too. But as I don't like Rogues and especially don't like dual wielding, and have no clue about the exact mechanics and damage calculation, I better stop here.
So, to sum up, weapons and damages in DnD/BG3 are not easy, but you get used to it quickly. You can see the proficiencies in the window which pops up with "I", on the left side. You also see the damages and modifiers under the character.
