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. 

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