If you define what kind of thing you want to build, it might be easier to gage in what to learn.

For example lets say I want to create a Bitcoin watchOS wallet that can receive, send , and connect to a remote blockchain.

Now you have to find the libraries you might be able to use to create Bitcoin wallets Like ‘Bitcoin Dev Kit' or Lightning Dev Kit

What languages do they code in might be the next step.

They use rust, and have language bindings for IOS to build iPhone apps.