The PageModelService defines the functionality and behaviour required by your Razor PageModel. So in the case of a Customer Razor page, the PageModelService could implement functions sush as GetOrderList(), UpdateProfile() etc. The PageModelService is implemented as a reference on the PageModel class.
So the PageModelService doesn’t receive any services, but rather it defines them.
Does that make sense? If not I could do a follow up article and give a more detailed example.