if you are stunned, dazed etc then that adds a debuff... if you have advantage {high ground or spell} then that adds an extra dice i.e, you roll 2D20 and pick the best result from them to do your math
the example you show is a longsword so D10 if you have just that equip and D8 if you have something in the off hand.. i see red_Queen covered this
spells bounce off most bosses unless you debuff them some how a Hex from Wyll or use a caster with 18 or 20 in their main casting stat so that adds +4 [18] or +5 {20} to your roll against their save
p.s, “Finesse” don't have to use dexterity instead of strength... its just they can if 1 is higher i.e, a str based character can still use a “Finesse” weapon
