variable that's not declared works without error why? - How Does property - method work (Get Set)

I'm learning the locomotion and this script bugges me:

PlatformCharacterController script changes

desiredMovementDirection in this script:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(Rigidbody))]
 [RequireComponent(typeof(CapsuleCollider))]
 public class PhysicsCharacterMotor : CharacterMotor {
     
     public float maxRotationSpeed = 270;
     public bool useCentricGravity = false;
     public LayerMask groundLayers;
     public Vector3 gravityCenter = Vector3.zero;
     
     void Awake () {
         rigidbody.freezeRotation = true;
         rigidbody.useGravity = false;
     }
     
     private void AdjustToGravity() {
         int origLayer = gameObject.layer;
         gameObject.layer = 2;
         
         Vector3 currentUp = transform.up;
         //Vector3 gravityUp = (transform.position-gravityCenter).normalized;
         
         float damping = Mathf.Clamp01(Time.deltaTime*5);
         
         RaycastHit hit;
         
         Vector3 desiredUp = Vector3.zero;
         for (int i=0; i<8; i++) {
             Vector3 rayStart =
                 transform.position
                     + transform.up
                     + Quaternion.AngleAxis(360*i/8.0f, transform.up)
                         * (transform.right*0.5f)
                     + desiredVelocity*0.2f;
             if ( Physics.Raycast(rayStart, transform.up*-2, out hit, 3.0f, groundLayers.value) ) {
                 desiredUp += hit.normal;
             }
         }
         desiredUp = (currentUp+desiredUp).normalized;
         Vector3 newUp = (currentUp+desiredUp*damping).normalized;
         
         float angle = Vector3.Angle(currentUp,newUp);
         if (angle>0.01) {
             Vector3 axis = Vector3.Cross(currentUp,newUp).normalized;
             Quaternion rot = Quaternion.AngleAxis(angle,axis);
             transform.rotation = rot * transform.rotation;
         }
         
         gameObject.layer = origLayer;
     }
     
     private void UpdateFacingDirection() {
         // Calculate which way character should be facing
         float facingWeight = desiredFacingDirection.magnitude;
         Vector3 combinedFacingDirection = (
             transform.rotation * desiredMovementDirection * (1-facingWeight)
             + desiredFacingDirection * facingWeight
         );
         combinedFacingDirection = Util.ProjectOntoPlane(combinedFacingDirection, transform.up);
         combinedFacingDirection = alignCorrection * combinedFacingDirection;
         
         if (combinedFacingDirection.sqrMagnitude > 0.1f) {
             Vector3 newForward = Util.ConstantSlerp(
                 transform.forward,
                 combinedFacingDirection,
                 maxRotationSpeed*Time.deltaTime
             );
             newForward = Util.ProjectOntoPlane(newForward, transform.up);
             //Debug.DrawLine(transform.position, transform.position+newForward, Color.yellow);
             Quaternion q = new Quaternion();
             q.SetLookRotation(newForward, transform.up);
             transform.rotation = q;
         }
     }
     
     private void UpdateVelocity() {
         Vector3 velocity = rigidbody.velocity;
         if (grounded) velocity = Util.ProjectOntoPlane(velocity, transform.up);
         
         // Calculate how fast we should be moving
         jumping = false;
         if (grounded) {
             // Apply a force that attempts to reach our target velocity
             Vector3 velocityChange = (desiredVelocity - velocity);
             if (velocityChange.magnitude > maxVelocityChange) {
                 velocityChange = velocityChange.normalized * maxVelocityChange;
             }
             rigidbody.AddForce(velocityChange, ForceMode.VelocityChange);
         
             // Jump
             if (canJump && Input.GetButton("Jump")) {
                 rigidbody.velocity = velocity + transform.up * Mathf.Sqrt(2 * jumpHeight * gravity);
                 jumping = true;
             }
         }
         
         // Apply downwards gravity
         rigidbody.AddForce(transform.up * -gravity * rigidbody.mass);
         
         grounded = false;
     }
     void OnCollisionStay () {
         grounded = true;
     }
     
     void FixedUpdate () {
         if (useCentricGravity) AdjustToGravity();
         
         UpdateFacingDirection();
         
         UpdateVelocity();
     }
     
 }

and I don't understand how can it change it if it's not declated it's not public nor even private

I did Ctrl + F and found only and only 1 through whole script.

this bugges my whole basic understanding of programming.

I need some explanations here thanks.

PS. what am I doing: I don't know --> (learning and combining my programmed stuff with locomotion).

I changed the title since it was clearly explained how property works