Migrating html to wordpress: what is the “go live” step?

“Going live” is a rather subjective expression. It usually refers to the moment when a website is accessible to the public. You determine this moment yourself.

For example, you can prepare everything under the future live domain and exclude the public in a maintenance mode or password protection. Removing this mode or protection celebrates the “going live” moment.

However, you can also prepare everything in a non-public staging and transfer the data from there to the future live hosting for going live. You could then proceed as described here: https://developer.www.ads-software.com/advanced-administration/upgrade/migrating/#moving-to-a-new-server

What I want to know is what technical step is the one that makes the new content accessible to all when moving to production – is it the url setting?

As described above, it all depends on how you hide the content from the public beforehand. You can prepare the project under a different domain than it should have later. Then going live with a change of domain would be feasible. There are also instructions for this: https://developer.www.ads-software.com/advanced-administration/upgrade/migrating/#changing-the-site-url