I don’t run very often, but when I do, I don’t want to carry my huge OnePlus 8 Pro with me - luckily I have a Fossil Carlyle, too. The watch does have internal memory and I can connect bluetooth headphones to it, but Android Wear is lacking a good way to transfer music files easily. The only real way to have music on the watch was a Google Music subscription, which allowed offline storage of music on the watch - but Google Music was shut down months ago and there is no replacement. Spotify can not store files on the watch and I doubt they will support this in the future - in general there are no new features coming to Spotify at all.
Yesterday I wanted to have at least one short playlist on my watch to have a backup when I only want to carry the watch with me and still listen to music. After a quick search, the App WearMedia came up in my search results.
After installing the app on my phone and my smartphone I tried to transfer my go-to cardio playlist from the phone to my watch. The first tries didn’t really work and I was really confused, until I realized I need to give the watch app permission to access storage - the permission dialog came up after I started the watch app for the first time. Make sure, you open the watch app at least once before you try to transfer files.
After that, it was pretty simple: Open WearMedia on my smartphone, browse storage to find my media files, select a folder or individual files and press one button. The transfer rate is pretty slow but I just let my phone sit on my desk and a couple minutes later the files were available on my watch.
I attached my trusty SBH24, opened the watch app of WearMedia and was able to listen to my cardio playlist without my phone around.