As stupid as the title of this thread sounds, 95% of all tutorials of Plex on the Raspberry Pi or other general armhf / arm64 devices are garbage and completely broken. A simple indicator is the use of the file
/etc/default/plexmediaserver.prev in the tutorial. This file is a leftover backup file and has no affect at all on your system or on Plex. Any tutorial mentioning this file and a way to change the Plex user e.g. to
pi via this file are a waste of your and my time. Please ignore these tutorials completely, the authors clearly do not know what they are talking about (and copying one from another). Please also leave appropriate comments to urge the authors to fix their stuff.
Then please continue to read our official setup instruction here in the forum. This document provides a great starting place and pointers to other Plex Linux guides discussing the mounting of USB or network drives and so on.