Sphere and spotlight

Does DS rely upon Spotlight? Or one can disable SL indexing…?

It’s doesn’t rely on it but it does it use for some formats.
Why would you disable Spotlight?

it seems to me that SL never ends to index; sort of perpetual spinning wheel…

Maybe try rebuilding the Spotlight index: Rebuild the Spotlight index on your Mac - Apple Support

thanks, exactly what I have done…
Where does DS put the index files, in the Application Support folder?

Yes, the indexed data is stored ~/Library/Application Support/DEVONsphere Express/.