I want to learn how to design a simple current source using a BJT.

I found this picture on another site (https://www.wellpcb.com/blog/pcb-projects/constant-current-source/?utm_source=chatgpt.com), but it doesn't seem to say how I'm supposed to choose the component values, and I haven't had much luck in finding other sources that do.

I assume R1 is chosen such that the Zener is in its breakdown region, but how do we choose Re?

What is Re's role here?

Does this circuit work for a larger range of input voltages?