I'm trying to pull the components (fields/objects....) from the un-managed package but I could not find any way using APIs or APEX is that even possible ?
Here is the example:
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Yes, sfdx force:source:retrieve has a "package name" option:
sfdx force:source:retrieve -n "My Package Name"
You can also retrieve a package using the MDAPI by specifying a package name (e.g. via the Ant Migration Tool, Workbench, etc). DX has an MDAPI option, too:
sfdx force:mdapi:retrieve -p "My Package Name"