How does Google's tag system work?

Google Tag Manager or any other tag manager works in the same way.

Enter the tag manager, it will ask your developer to place only one piece of code on the website that is the tag administrator's code. That is all. You can manage all your marketing and analytics tags in the Tag Manager console.

In summary, Tag Manager is a cloud-based tool where you can manage JS tags by yourself without the help of the developer changing the source code of your page.