Swift is a modern and versatile object-oriented language. It is full open source and parts of Apple’s Xcode IDE are also open source. Apple’s system libraries are proprietary - no surprise.
Having learned coding in C at university (many years ago) makes it easy for me to get familiar with the Swift syntax. However, things have changed since my C-coder days. Swift is OOP and offers advanced features that make it easier (no more manual allocation and release of memory - malloc()) and more challenging (many more things to memorise and to put in action while coding) at the same time.
Some people do learn from webinars, videos and online courses. Others like me do learn better and faster by using monographs and references.
Well-written monographs can save a lot of time by presenting the large amount of technical detail in context and pointing at what is important to consider.
I am fairly easy with English but I can go faster with a monograph written in my native language, German.
For learning Swift, these two monographs, in German, I recommend:
https://fachbuch.hanser-ebooks.de/ebook/bid-3095195-das-swift-handbuch-apps-programmieren-fuer-macos-ios-watchos-und-tvos-inkl-updates-zum-buch.html
Both provide a comprehensive overview of Swift and working with Xcode. The publisher’s webpages offer reading samples.
If you have other monographs you’d like to recommend, please add info about the language its written in.