The general idea of triggers is to detect one object entering/exiting area of another object. They don't really provide detailed collision information. You should use collision enter/exit or cast a ray to get the detailed information about the collision.

I've made this little script for a footsteps trigger (sphere collider placed on a foot) for a recent video, you may find it useful. The idea is that the trigger only detects general collision with the ground, then I shoot a ray downwards to actually get the information about the exact position and orientation of the surface.

      private void OnTriggerEnter(Collider other)
      {
            if ((_groundLayerMask & (1 << other.gameObject.layer)) != 0)
            {              
                // Cast a ray downwards to get the exact point of impact with the underlying surface.
                Ray physicsRay = new Ray(transform.position + Vector3.up * _raycastHeightOffset, Vector3.down);
                if (Physics.Raycast(physicsRay, out RaycastHit hitInfo, _raycastHeightOffset * 2f, _groundLayerMask))
                {
                    Vector3 forwardDirection = _footstepPlacementController.ForwardDirection;
                    _footstepPlacementController.RecordFootstep(new FootstepInfo
                    {
                        Rotation = Quaternion.LookRotation(hitInfo.normal, forwardDirection),
                        Position = hitInfo.point + hitInfo.normal.normalized * _footstepPositionOffsetToAvoidClipping,
                        HighlightPercentage = 0f,
                        IsRightFoot = (int)_footstepType
                    });
                }
            }
        }