What is default "fstab" file in Android?
It can be in / (ramdisk), /odm/etc/ or /vendor/etc/.

Android Storage Configuration:


  For Android releases 4.3 and later, the various fstab files used by init, vold and recovery were unified in the /fstab.&lt;device&gt; file


Next change occured with Treble when vendor/SoC specific code was separated from Generic AOSP code. So fstab was moved to /odm or /vendor.

default_fstab_path reads GetFstabPath():

// Identify path to fstab file. Lookup is based on pattern fstab.&lt;hardware&gt;,
// fstab.&lt;hardware.platform&gt; in folders /odm/etc, /vendor/etc, or /.


Commit states:


  fstab contains device- and soc- specific content that should reside in
  /odm or /vendor partition. This change searches the fstab.${ro.hardware}
  file from /odm/etc, /vendor/etc and /, then use the first one found.


fstab contains entries for all filesystems to be mounted on boot or in recovery from partitions including system, vendor, userdata, cache, misc, persist, modem, dsp etc. See details in Android Partitions and Filesystems.

Some of the entries of fstab are moved to DTB. See "WHERE FSTAB IS?" in How to disable dm-verity on Android?