Testing is the key to success
This doesn’t mean that’s it’s all automatic. There’s always some kind of developer intervention or investigation if the code doesn’t pass the tests. And some tests can’t be automated, like checking that everything looks as expected on a website, checking if a button is in the right place on the page and that the click works.
So there are different ways to test and release code, but you should always check your site to make sure it looks right.