Improvised weapons are not ranged weapons or melee weapons, but they do make ranged weapon attacks and melee weapon attacks, and you add damage modifiers to such attacks. Improvised weapons are also not shields.

What weapons are simple weapons?

Weapon Proficiency

Most people can use simple Weapons with proficiency. These Weapons include clubs, maces, and other Weapons often found in the hands of commoners. Martial Weapons, including Swords, axes, and polearms, require more specialized Training to use effectively.

Are improvised weapons strength based?

Yeah, improvised weapons don't have the finesse property, so throwing the flask is strength to hit. Ranged weapon attacks use DEX. The only reason melee weapons with the Thrown property can use STR when thrown is because the Thrown property says so.