I think that for watching content saved to the phone "VLC" is the way to go. However, I personally use "Emby" when watching on iOS and I don't use the app (because they charge money for it) and the web-app version works just fine. This is my go to when streaming media server content to my phone, I've used plex and jellyfin too. Jellyfin is nice because it split from emby after they decided to start charging but they don't support all smart TV's. Plex is on everything I think but it has always felt bloated to me. I'll shoot you a DM with an emby account log in so you can check it out if you want.