When was the last time you used the clusterf* that is iTunes on windows?

Or more generically answer the question: how can I get an arbitrary audio file into my iTunes music? (hint: good luck)

Music 'synced' with iTunes but not appearing on my other devices? There must be some kind of arbitrary difference between 'synced with iTunes' and 'synced with iCloud'. I guarantee this is some kind of (barely) maintained legacy syncing to keep the iTunes workflow alive specifically so Apple can avoid giving users a modern 'import to my cloud library' feature.