Yes, as Ingo said: the OSPi schematic assumes it’s LM2596-5.0, which outputs a fixed 5V. Sorry that the schematic caused confusion — but it’s good for you to know that most switching regulators come with either fixed voltage version, or adjustable voltage version. If you use adjustable voltage version, you need to supply two resistors which sets the output voltage.
The zener is used for additional protection — if someone puts a high voltage directly on the 5V line, that could damage the circuit. It’s optional but just there for additional protection.