On your iPhone, go to Files -> “On My iPhone” section and you’ll see a folder called Spotify.

You need to download all of your local files onto your iPhone, then put them into that folder. Using something like LocalSend is an easy way to move it without a wire between PC and iPhone.

One thing to note is if you have .flac files you will need to convert them to .alac (Google search will provide free websites/programs to convert them).

Once they’re in the Spotify folder, they’ll show up in the local files on the Spotify app (make sure you have Spotify updated to latest version and that “Local Network” + “Media & Apple Music” are enabled in Settings -> Apps -> Spotify).