The Player’s Handbook, Chapter 9, states about attack rolls:
The ability modifier used for a melee weapon attack is Strength
A bit later on it also states about melee attacks:
Instead of using a weapon to make a melee weapon attack, you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow (none of which count as weapons). On a hit, an unarmed strike deals bludgeoning damage equal to 1 + your Strength modifier. You are proficient with your unarmed strikes.
I’m not sure whether an unarmed strike is considered a “melee weapon attack.”
For a basic unarmed strike (i.e. no Monk, Tavern Brawler or anything else that enhances unarmed strikes), is only the proficiency bonus added to the attack roll, or is the Strength modifier also added?
Clearly the Strength modifier is added to the damage, but I’m asking about the attack roll.