I don't know about Swiftfin, but the Jellyfin Mobile app is a wrapper of the web UI which allows you to download content. But you need to use another player like VLC to watch that media offline.
Jellyfin 10.10.0 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)
No. You use jellyfin to download the video file. It will be stored in your on your phone (on Android it goes in the main Download folder). You use VLC's UI to open the video file in the download folder.
Jellyfin 10.10.0 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)
We have a long flight coming up and I realised that a feature like this is really missed. I can access my server remotely, but when there's no service, we're out of luck. I really do hope that it's in the works at some point. My kid would be so stoked.
I would also recommend Infuse for iOS. It's pretty nice and plays well with Jellyfin with a super straightforward setup. I've also got a cheap Amazon fire tablet (that I've installed the Play Store on) that we use for travel. It's the only device we have that isn't in Apple's ecosystem, and I was really disappointed to not be able to find Infuse on the play store. I'd be curious to know if there's an an equivalent for non-iOS devices that would allow downloads for offline viewing.