You can learn about another creature’s resistances or vulnerabilities as a bonus action, but only once per long rest – unless you expend a superiority die. Relentless lets you use a Maneuver without using a superiority die, but with a smaller die roll. The Maneuvers from Tasha’s Cauldron...
From level six, their ki begins to empower their blades. Kensei weapons are treated as magical for the purpose of enemy resistances, and they can spend ki to use a Deft Strike once per turn, adding an extra Martial Arts die in damage to the attack. ...