In general I would advise not to actively use 'signals' or 'slots'. Since they are macros there is a risk for that to collide with third party libraries and to avoid problems here you can switch these macros off by configuring Qt accordingly. Instead, you should use 'Q_SIGNALS' and 'Q_SLOTS' which, while still macros, are not of such a risk of namespace collisions.
Also, for the purpose of understanding Qt at a deeper level I would forget about designing using .ui files. They don't allow you to do anything you could not also do by hand but they hide a lot of the details. That's good when you need to get something done quickly, it's bad when you really want to understand what is happening. Try building a few UIs yourself. Create layouts, add child widgets to them and research which signal/slot connections you need to get something done. That will be slow going at first but it will force you to do a lot of research in the documentation. Liberally read that, things linked there, as well as the introduction of Qt concepts.