I am confused after reading codes of PostgreSQL three week - Mailing list pgsql-hackers

Hi,
 I try to known how a database is implemented and I have been reading
PG source codes for a month.

Now, I only know a little about how PG work.  :(

I just know PG work like this but I don't know why PG work like this.  :(  :(

even worse, I feel I can better understand the source code. it may be
that I could't split the large module into small piece which may help
to understand.

Is there any article or some way could help understand the source code ?

Thanks for help ~

--
Best Wishes!

                                     hom