Find the shortest path between 2 points in a mesh

Good morning,
premising that I am completely new to 3d programming the only knowledge I have are the studies I did at university more than 20 years ago..
I would like to develop a basic app able to import 3d mesh data from ply files and, after taking 2 points on the surface, should be able to find the shortest path between them.

I started modifying the basicshapes-cpp Qt demo to import the polygon data from an external ply file instead of creating the shapes from coding.
The basic app compile and run successfully, but when I load a shape from the files I have, I can not see it centered or I see it too small.

Here the code I've used (code modified from basichapes-cpp demo):

Code:

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    Qt3DExtras::Qt3DWindow *view = new Qt3DExtras::Qt3DWindow();
    view->defaultFrameGraph()->setClearColor(QColor(QRgb(0x4d4d4f)));
    QWidget *container = QWidget::createWindowContainer(view);
    QSize screenSize = view->screen()->size();
    container->setMinimumSize(QSize(200, 100));
    container->setMaximumSize(screenSize);
 
    QWidget *widget = new QWidget;
    QHBoxLayout *hLayout = new QHBoxLayout(widget);
    QVBoxLayout *vLayout = new QVBoxLayout();
    vLayout->setAlignment(Qt::AlignTop);
    hLayout->addWidget(container, 1);
    hLayout->addLayout(vLayout);
 
    widget->setWindowTitle(QStringLiteral("Basic Mesh Viewer"));
 
    Qt3DInput::QInputAspect *input = new Qt3DInput::QInputAspect;
    view->registerAspect(input);
 
    // Root entity
    Qt3DCore::QEntity *rootEntity = new Qt3DCore::QEntity();
 
    // Camera
    Qt3DRender::QCamera *cameraEntity = view->camera();
 
    cameraEntity->lens()->setPerspectiveProjection(45.0f, 16.0f/9.0f, 0.1f, 1000.0f);
    cameraEntity->setPosition(QVector3D(0, 0, -1.0f));
    cameraEntity->setUpVector(QVector3D(0, 1, 0));
    cameraEntity->setViewCenter(QVector3D(0, 0, 0));
 
    Qt3DCore::QEntity *lightEntity = new Qt3DCore::QEntity(rootEntity);
    Qt3DRender::QPointLight *light = new Qt3DRender::QPointLight(lightEntity);
    light->setColor("white");
    light->setIntensity(1);
    lightEntity->addComponent(light);
 
    Qt3DCore::QTransform *lightTransform = new Qt3DCore::QTransform(lightEntity);
    lightTransform->setTranslation(cameraEntity->position());
    lightEntity->addComponent(lightTransform);
 
    // Loading .ply data
    QUrl data = QUrl::fromLocalFile("monster.ply");
 
    Qt3DRender::QMesh *bodyMesh = new Qt3DRender::QMesh();
    bodyMesh->setMeshName("bodyMesh");
    bodyMesh->setSource(data);
 
    Qt3DCore::QTransform *bodyTransform = new Qt3DCore::QTransform;
    bodyTransform->setScale3D(QVector3D(10.0, 10.0, 10.0));
 
    Qt3DExtras::QPhongMaterial *bodyMaterial = new Qt3DExtras::QPhongMaterial();
    bodyMaterial->setDiffuse(QColor(QRgb(0x928327)));
 
    Qt3DCore::QEntity *plyEntity = new Qt3DCore::QEntity(rootEntity);
    plyEntity->addComponent(bodyMesh);
    plyEntity->addComponent(bodyMaterial);
    plyEntity->addComponent(bodyTransform);
 
    // Set root object of the scene
    view->setRootEntity(rootEntity);
 
    // Show window
    widget->show();
    widget->resize(1200, 800);
 
    return app.exec();
}

I would like to have the following help:

Quote:

* Understand why I see the shape infinitely small and how to fix it
* Understand how I can center the shape in the screen and big enough so I can see it well
* Understand how to add mouse interaction so I can zoom/move the scene
* After a mouse left click, understand how I can get the coordinates of a specific polygon so that a shortest path algorithm can be applied.

Thank you very much