How does Google Tag Manager extract tracking data from datalayer?

@virginie 

Google Tag Manager (GTM) uses a data layer to extract tracking data from a website. The data layer is a JavaScript object that contains information about the user, their behavior, and the context of their visit to the website. When the data layer is implemented on a website, it can be used to trigger tags, which are snippets of code that perform specific actions, such as tracking user behavior.

To extract tracking data from the data layer, GTM uses built-in variables and custom variables. Built-in variables are pre-defined variables that are automatically populated by GTM with data from the data layer. Custom variables, on the other hand, are variables that you define in GTM and populate with data from the data layer.

Here's a step-by-step process for how GTM extracts tracking data from the data layer:

1. The data layer is implemented on the website using JavaScript code.
2. When a user interacts with the website, data about their behavior and context is added to the data layer.
3. GTM is installed on the website, and the GTM container code is added to the website's header.
4. When a user interacts with the website, GTM reads the data in the data layer and populates built-in variables and custom variables with relevant data.
5. Tags are created in GTM that are triggered by specific events, such as when a user clicks on a button or submits a form.
6. When a tag is triggered, GTM uses the data in the built-in and custom variables to populate tracking parameters, such as page views, button clicks, or form submissions.
7. The tracking data is then sent to third-party analytics tools, such as Google Analytics, for analysis and reporting.