Hi aldessa and welcome to the forums! You won't be able to link your games on the Switch and an iOS device for a couple of reasons. First is that the Switch has the 1.5.4 version of Stardew Valley while iOS (and Android) still are stuck on the 1.4.5 version of the game and you can't interchange save files in these different versions.
If they ever get updated to the same version, the second issue is that there's no easy way to move these game saves between these 2 platforms as that functionality isn't built in to the Stardew Valley app. However, there are third party tools to access your save file on the Switch, and accessing your save on iOS requires doing a lot of copying and saving through iTunes. So it can be done (once the versions are all the same) if you have the patience for it.
I sure don't. Which means I can just start a new Farm on a new map and do it all a different way!