Where are the detoast function called in select * from table_name case?

Hello,

I've been reading the PostgreSQL code for weeks to figure out how TOAST 
works.
I couldn't find where are the TOAST function called to detoast a tuple 
comes from a select query, for example, select * from table_name.
Does anyone know this? Can you give me some help? And any help would 
save me a lot of time.

Thanks,
Rui Hai