Starting issues after repairs, new serpentine, ICM, H2O pump, coolant thermostat, fa n relay. What would cause this? 06 Pontiac Grand Prix?

Water pumps fail. It happens. If an engine overheats several times or if it overheats drastically once it can cause damage to those other things in the coolant system. It's not impossible that they were all bad, all failed at the same time, or the mechanic replaced all it to insure all of the coolant stuff was new and working properly. It's also not impossible that they replaced all of that to dirty up the bill but even if they did that...you have all new coolant parts. You would have reason to complain if you knew for sure some of it didn't have to be replaced but without proof you should not complain. You have all new parts and it should be working properly again.

You expect someone online to troubleshoot a car with "starting issues"? How about a longer story than that.  Tell us a longer story.  Does the starter crank?  Does it crank but does not start?  Did you try to get a jump?  Etc, etc.

So many things could do that, it would not serve you at all to list them, especially since an OBD2 code check will point right to where the problem should be diagnosed. None of those repairs should affect starting. Think back over the situation around getting all that work done: Did the engine overheat, or did you just get all those repairs done because the car is old? Overheating can do certain damage which needs mechanical repair.