Help about physical page layout in PostgreSQL:

Hi,

I was trying to modify the physical page layout in PostgreSQL. I understand
the source code for physical page organization is located at bufpage.c but I
wonder what is the external interface for this physical page organization.
Interface like InsertRecToPage, DeleteRecFromPage, GetAttr, GetRec, etc?

My question are 1.where are the source files for these interfaces to
physical page?
2. Are there more detailed technical documentation for this kind of stuff?
The documentation at the website is more llike a external description of the
whole system. Is there any technical documentation for the code?

Thank you for your time, any help is appreciated!
Best,
Yeye