If you really need syncing, then just maybe use the free version of Dropbox? Unless you have a lot of large projects that you are always working on, this should work for most.
Otherwise if you don’t really need to sync, and copying the project around is just fine, there are ways of doing that (further down in that thread, I collected some resources for other alternatives, too). The “iTunes” hint is one method, though that label is outdated,[1] and all it really means is to refresh the software against what is stored in its files area. You can look at that yourself on the device, using Files.app. If you put a project in there, Scrivener might not immediately see it, and that is what the “update from iTunes” button would have it check.
But these days there are more convenient ways of doing that than plugging in, including using iCloud Drive.
This comes from the days when iTunes still existed, and was how you managed your mobile devices. Since originally that mean iPods, it made sense at one point. ↩︎