Fetching Values of session defaults

With the new introduction of session defaults ,

How are the session defaults being set ?
are they being set on all the Link fields of the document set on defaults?

How can i access the session defaults programmatically ?
I cant see any difference in frappe/sessions.py ?

where are the variables set , and how can i fetch them on custom fields ?

Any idea from any of the developers in the whole community ,

I am going through the codes but not finding clear guides or path ,

Any help might be appreciated ,.