Yes you could start from one of the examples in the FW package; In SW4STM32 you can create a project selecting the NUCLEO-H755ZI-Q board for example (or create a custom board), then, on the firmware selection page, select the “Extract all firmware in separate folder” check-box.
You will then have a folder in your workspace from which you can import the example code and play with it.
To create a new project, you can start from this example project or populate your own project. To access the GPIO, you should go through the HAL firmware; again examples are a great source fo inspiration. For the encryption module, probably you should look in more detail at the firmware and chip documentation; it may be a bit tricky.
Both C and C++ are supported and can be used freely. ST firmwares are writtent in C but meant to be usable from C++.