Hello @Samy Abdul and welcome to Microsoft Q&A.
I feel there is some degree of confusion among whether a variable belongs to the devops pipeline , or the Data Factory pipeline. There are many words being overloaded here. While devops and DataFactory both use the words "pipeline" and "variable" and "global", they are not the same.
In Azure Data Factory, a Global Variable, is a key-value pair made available in the context of all ADF pipelines in that Factory. Any activity can have a Dynamic Expression which makes use of the Global Variable. The activity can make use of the Global Variable, including passing the value down to a parameterized Dataset, and the Dataset pass it down to a parameterized Link Service.
The devops variables and parameters are used mostly in devops. The only way they can touch upon Data Factory, is during the deployment process. An ARM template can have parameters. Devops can provide values for the template parameters. Most commonly, this is used to point a linked service to the correct resource / environment, such as having a test database separate from a production database.
How the ARM template is parameterized determines what can change in the Data Factory during deployment. I think Data Factory Global Variables can fall under this category.
Does this help or make sense?