[NEST Users] how to customize Ion channels and receptor types of "cm_default" neuron model through nestml

Dear nest community, I wonder if any of you know how to customize Ion channels and receptor types of "cm_default" neuron model through nestml. I noticed some information on Extending NESTML- Running NESTML with custom templates, but I still found myself confused due to my poor understanding. Is there any way to acquire the .nestml file of the cm_default neuron model? Thank you. Best, Zirui