How does Postgres store a B-Tree on disk while using the OS file system?

[Siddharth Jain <siddhsql@xxxxxxxxx>]

I am trying to sharpen my understanding of Postgres. As I understand, Postgres does not write directly to disk blocks. It uses the file system provided by the OS:

https://dba.stackexchange.com/questions/80036/is-there-a-way-to-store-a-postgresql-database-directly-on-a-block-device-not-fi

My question: How can it then store a B Tree on disk? I would think storing a B Tree requires storing disk offset addresses and so on (for a node to navigate to another etc.). For this, one would need to write directly to the disk using low-level disk access functions and not use file system API.