You cannot filter by certain codecs using Plex.
There’s a 3rd party app to export your library including the technical parameters, so you can filter that output file.
You shouldn’t rely on file names to show what’s in a file.
Side note: x264 and x265 usually refer to an encoder for the h264/h265 codecs respectively. The same way, HEVC is the abbreviation of the full name for h265.
