With some areas it will depend on any template that you use but it generally is set up for some sites by default through templates and if it’s not it is really easy to install. The same applies for WordPress sites, although it might not be there on the default install, in most cases there’s just a simple plug-in to use. On a new WordPress website for example there would be a single plug-in, and rather than having to copy and paste code to the specific files directly, the plug-in would do that for you.
So it’s very straightforward and easy to implement Google Analytics and tracking scripts, and there’s lots of ways of doing it. But I would always go back to and follow the step-by-step instructions provided by Google because it does change. I’d go to support.google.com because they tend to be pretty good at updating things.
I’ve also looked at Shopify who make things really easy now, where they’ve just created an “out of the box” feature so you just put your GA tracking code in there and it does the rest.