I'm reading the basic books but this doesn't get explained. Let's have an example: a PC is attacking an AC16 orc with a sword (he's proficient in swords). He rolls a 13 + STR bonus (+ 2) + Proficiency bonus (+ 2) = 17, it's a hit. Now it's the orc's turn, he rolls a 10 (+ 2) STR bonus + ...proficiency bonus??
Yes, NPCs and monsters have proficiency bonuses too
Monsters have proficiency bonuses and get to use them when appropriate, just like player characters do. While a PC's proficiency bonus is based on their level, a monster's proficiency bonus is based on their CR, according to this table - from a simple +2 at CR 0 to a maximum of +9 for CR 30 creatures.
The stats blocks for published monsters will already be applying the monster's proficiency bonus where appropriate. For example, the CR1/2 Orc has a greataxe attack:
Greataxe. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (1d12 + 3) slashing damage.
That +5 attack bonus includes the Orc's +3 Strength modifier and their +2 proficiency bonus as a CR 1/2 creature.