Samsung Galaxy S5 - Is my phone's internal 16 GB memory on the SIM card or are they separate?

A SIM card is not storage. A SIM is the card needed for service with your carrier/service provider. Without it, no service. As apposed to a SD card which is something you purchase to add additional "external" storage to a device.

The storage within the phone (which is the "Internal" storage) is what you get when you purchase the device. However, not all that storage is available to you. That is the totally the device has. However, much of that is taken up by the OS and the preinstalled apps.