Many things cause left-turning tendency, and they all inter-relate.

P-factor, also called assymetric disc loading, is most evident during slow-speed, high angle-of-attack flight at high power settings. It's cause is from one side of the propeller disc producing more thrust than the other. In our case with clockwise-turning props (viewed from the cockpit), the right-hand side of the prop has a greater angle-of-attack, so the plane tries to turn left. Also called 'prop-walking' in boats

Gyroscopic precession, sometimes called torque precession, occurs when the prop is moved from its plane of rotation, and the effect occurs 90 degrees from where the force was applied. In a taildragger, as the nose is pitched down to raise the tail, the 'push' is applied at the top of the propeller and the resulting force reacts 90 degrees clockwise (again, from the cockpit) to the right side of the prop disc, causing a yaw to the left. The opposite occurs in nosewheel aircraft; as the nose is raised, there is a bit of right yaw but it's negligible as the other factors counter it.

Spiral slipstream effect is what GB correctly pointed out, the spiral propwash striking the left side of the fin and rudder, also causing a left yaw. The slower the airspeed, the tighter the spiral and the greater the effect. It's present even during cruise flight, and many single-engine full-size aircraft have offset vertical fins to counteract the effect.

Engine torque also has an effect, and it's more pronounced in full-scale aircraft. Quickly increasing the throttle can cause a roll and , in the clockwise example given, a left roll can result. More pronounced at very slow speeds. My Multiplex Funjet rolls left during handlaunch, and I've read that a fullsize Bearcat will roll over on its back if the throttle is suddenly advanced.

P-factor and gyroscopic precession are the biggest culprits for us. Add power slowly at takeoff (3-5 seconds from zero to full).  As the aircraft accelerates, the tail becomes more effective and will help keep it straight. Make slow pitch changes also, for the same reason.

On the more powerful fullsize twins I fly, adding power too rapidly will cause the aircraft to turn left and depart the runway. No amount of brake or right rudder will stop it. Slow, smooth inputs are the answer.