bgwriter and checkpoints

Hi community, I was reading about background writer process and
checkpoints, but I still don't understand very well. How do they work for
instance I understand this if a row was updated,those changes will be on
shared buffers, then those changes will go to path on disk called pg_wall,
then when a checkpoint will be executed and finally, those changes will go
to data files when bgwriter process will be executed but I found a blog on
internet they say that after each search, the BGWRITER chooses some number
of modified pages, writes them to disk, and evicts those pages from the
shared buffer pool.

https://flylib.com/books/en/2.290.1/the_postgresql_bgwriter_process.html

--
Ing. Henry G. Cortez