SUMMARY:
Elisa fails to index the system for audio files even when acceptable folders to look in is put in the app's settings and indexing is allowed in the system settings
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STEPS TO REPRODUCE [ this goes from the start of installation and was achieved on a Samsung Galaxy xCover 5 smartphone running Android 11 build number RP1A.200720.012.G525FXXS5AVB3 ]
1) Install the Elisa APK from the KDE Binary Factory (I downloaded the APK using Mozilla Firefox for Android)

2) Open the Elisa application and open the hamburger menu on the left side of the screen and click on "settings"

3) Scroll down to where it says "Music Folders" and click on "Add New Location" and select the file location of where your music is stored, for me the app displays the following path: content://com.android.externalstorage.documents/tree/primary%3AMusic
even if the actual file path selected in the file browser is /storage/emulated/0/Music according to the file browser "Total Commander"

4) Then close out of the settings page and in the hamburger menu, select any of the options that will try to index system files e.g. "Tracks" or "Files". The problem you'll get (or at least the problem I got) is a pink circle doing an infinite loop trying to load something but fails to actually load anything after minuets of loading, this has the same outcome whether the "Use fast native indexer" or the "Scan the filesystem directly" option is selected in the app's settings under the "Music Indexing" option.

Another potential variable is that my music folder has individual tracks that were downloaded from Soundcloud and tracks that are a part of albums which are in their own folders which were downloaded from Bandcamp both on a Windows 10 Computer (specifically an Acer Aspire XC-705 SFF Desktop but this info is irrelevant).
The file types in the music folder selected are JPGs, MP3s, WMAs, M4As, FLACs, (2) XSPFs (playlist files), (2) S3Ms (tracker/MOD files), plus a folder of a totally legitimate copy of Mike Oldfield's "Ommadawn" album which has the folder types .BUP, .IFO, .VOB plus there is a "hidden" /.thumbnails folder with 2 files one is a file called ".database_uuid" and another file called ".nomedia" 
Another factor is that I also have VLC Media Player installed which could be an issue if there are conflicting ffmpeg libraries installed on my device but probably not...
 
OBSERVED RESULT
No music is loaded and no files are visible

EXPECTED RESULT
Music & other files are loaded in the app namely the "Tracks", "Artists", "Albums" and "Files" section accessible via the hamburger menu

SOFTWARE/OS VERSIONS
Windows: n/a
macOS: n/a
Linux/KDE Plasma: n/a but this phone is running Android 11 with Google Play Services
(available in About System)
KDE Plasma Version: n/a
KDE Frameworks Version: n/a
Qt Version: n/a

ADDITIONAL INFORMATION
This is a Samsung Galaxy xCover 5 smartphone running Android 11 with One UI Version 3.1 on the Vodafone UK mobile network. The phone has at least 2GB of RAM and at least 16GB of internal storage.

I'm not sure about the specific application version but it was whatever the latest release of the Arm64 apk file on the KDE Binary Factory (or whatever the site directory is called) on Tuesday 8th March 2022 at some point in the late evening. I don't know the specific app version as the file I downloaded doesn't display an app version, nor is it listed in the app itself BUT the "Settings" app on my phone says under "App info" that the version of Elisa that was installed is "Version 21.07.70" which is not an option that can be selected on the KDE bugs report website (bugs.kde.org) so I don't know if I was supposed to download this version or not and expect it to work...
And yes I already knew that it wouldn't be fully featured as the Android version of Elisa is not at all publicised (for good reason it seems!) - William Clifton minds,com/wclifton968