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I have 20 thematic maps with different sources like CITY PLANNING OFFICE, ENVIRONMENT OFFICE, AGRICULTURE OFFICE etc.

Since I have many maps, I want to automate the data source citation of my thematic maps by simply having a code or process. I want that each map layout, the data source will change based on the map that is being viewed. This will lessen the burden because I am not having to manually type the data source for each map layout.

I saw someone discussed about variables and it got my hopes high but it didn't work for me because the layer variables cannot be used in text label in map layout. Only the Global and project variables were working.

You should use the layers Metadata for it. In the Layers properties, fill in Attribution - Title string like shown below (you may use the Url too, as well):

Then insert the following expression into your label item (note the output preview in the screenshot and the arguments description of the layer_properties function on the right, you may access layer name, feature count and many more, no need for custom functions):

layer_properties(layer_id, 'attribution')

The layer_id must be in single quotes, you may obtain it either from the layers variables (but you cannot copy and paste it from there) or via the python console (with the desired layer selected):

iface.activeLayer().id()

If the source is the layer name you could define your own custom function and use it as an expression in the label content dialog.

Click "Insert an Expression..." in the Main Properties of the label. Select the function editor tab and paste the following. Click "Save and Load Functions".

# Be sure to import iface from qgis.utils



from qgis.core import *

from qgis.gui import *

from qgis.utils import iface



@qgsfunction(args='auto', group='Custom')

def get_layer_name(feature, parent):

    return qgis.utils.iface.activeLayer().name()

Then back on the Expression tab of the "Insert an Expression..." dialog double click the get_layer_name function (under Custom) to add it to your expression.

After clicking OK, the name of active layer in your main QGIS window will appear in your label. You will have to refresh the Layout Manager to update the name after a different layer is selected.