Is watchOS 5 a software that is added to an Apple Watch Series 4?

Yes watchOS 5 is a version of the software used to run the Apple Watch. The most current version is actually watchOS 5.2.1. What is it that you are trying to do with Google? Some background will help in responding to your inquiries.

I'm not sure what you are speaking of, what is Google link? You can send and receive Messages on the watch, through your phone.

Yes it is a software update that applies to all Apple Watches except the original watch. You can use Google on your iPhone, but not on your Watch.