Monk in 5e may hold a "monk weapon" and then choose to attack with the weapon or use their unarmed attack. The monk weapon will do damage equal to its usual damage die, or equal to the monk unarmed damage die, whichever is larger (this makes quarterstaff or spear very good weapons for low level monks, since they have d8 damage which is better than unarmed damage until level 11 or so).
When a monk takes the attack action with either their equipped weapon or unarmed attack, they may also take a bonus action to make an additional unarmed attack. If they spend a ki point they may make two bonus action unarmed attacks rather than just one. The bonus action attacks are unarmed even if the attack action used the weapon.
The above is how it works in 5e. Larian may have modified things a bit. If great sword can really be used as monk weapon, then it will pretty much always beat the unarmed damage die, unless Larian buffed that too.
