https://www.postgresql.org/docs/current/storage.html
https://www.postgresql.org/docs/current/storage-page-layout.html
https://postgrespro.ru/docs/postgrespro/current/storage-page-layout

each table consists from multiple layers
each file is no more than 1 Gb

object layers
?init?
    zagotovka of main
    used only while "sboy"
main
    NNN.2
    NNN.1
    NNN
fsm (free space mem)
    NNN_fsm
    NNN_fsm.1
vm (visibility map, needed for vacuum optimization)
    NNN_vm

Each file is a set of pages
page
    header
    [some free space]
    data

Buffer cache (caches some pages)

note
    on index-only scan we can't just use index (need xmin, xmax)
    visibility map allow us to overcome this limitation  

# cmd-line util
$ oid2name 

pg_total_relation_size(<tblname>)
pg_relation_size(<tblname>, <layername>)
pg_indexes_size(<tblname>)