Resources
The most important site would be the Apple Developer site. WWDC videos are essential. In addition to Apple's documentation, I've found these useful.
Recommended Websites
In addition to 100 Days of Swift, Paul Hudson has written 20+ books on coding in Swift, Objective-C, watchOS, iOS, macOS and more. The books are revised with new releases and are provided in several formats. I've put them all in a Devonthink database so I can easily search the books for programming advice.
There is a paid serivce, Hacking with Swift+ (HWS+), that ads videos and articles covering more advanced topics. I found the Ultimate Portfolio App series to be particularly useful when developing my first non-trivial application.
Kodeco was formally Ray Wenderlich and it contains books, articles and videos. The typical style is a tutorial. One of the nice features of the site is their Learning Paths where several courses are bundled into a coherent path. My recommendation would be to try it for a month. There is a reduced rate for the basic materials. Some of the preseenters and authors worked better for me than others, so watch a few to decide.
Point-Free is a tutorial site by the people who are behind the Composable Architecture. It is styled as pair-programming, typically with a Xcode Playground and is intended to bring understanding more than just introduce a way to do something. The journey is a big part of this site. Some of the series I've particularly found useful were the Functional Programming, Concurrecy and Modern SwiftUI ones.
This is another pair-programmer site with tutotials that attack a coding problem from multiple points of view before arriving at an accetable outcome. It feels like many of the conversations I've had in the past over a whiteboard when trying to solve something.
NSScreencast is a tutorial site that will spend time making apps or talking about strategies for working with a team of developers. I've found lots of useful content here.
Educative offers courses bundled up into learning paths. They are web-based and have integrated tests and exercises. There are no videos, it is all text and images, which makes it ideal for refreshing stale areas. There are good courses for preparing for coding interviews and general algorithms as well as language specific courses for C, C++, Python, JavaScript, etc. There is no swift or Apple specific content on this site.
Blogs
Podcasts
YouTube Channels
Azam makes nice tutorials. He also published Udemy courses.
Mikaeala builds apps in real-time during a recorded live-stream. If you want to see how an app gets made, this is a good one.
Sean Allen talks quickly and covers a number of essential introductory to intermediate level material. He also gives industry advice from his perspective. He has some courses on Teachable.
Stewart's channel is methodical and has nice examples. He covers a broad range of topics.
Tunds makes excellent videos that range from an introduction to SwiftUI to creating a caching asynchronos image view.
Vincent content dips into some more advanced concepts but also includes interview questions and 90 second introductions.
Rick Twohy documented his creation of a game engine in Metal. While there are some quirks (such as using a OpenGL style right handed coordinate system), he presents in a style that might help some overcome resistence to learning Metal.
If you enjoy watching time lapsed development videos of SwiftUI magic. This is the channel. He also has a Patreon with pre-release material and source code.
Every week another C++ topic is explored. Jason Turner's mission is to make C++ fun and accessible. Yeah!
Prof G has some amazing content. If you want to review Calculus (single and multi variable) and linear algebra, his courses do it in a visual and fun way. It is tragic these are not viewed more.
This channel is a retired math professor reading his course notes and I find it well paced and interesting. The notes can be purchased or you can read along with the videos.
His Transision to Advanced Mathematics course is great. You can also get a more mathematical foundation is Catagory Theory and Linear Algebra.
A visually based channel that covers math topics. The [Essence of Linear Algebra](https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab) series is helpful if you are attempting some 3d graphics.
If you are interested in graphics and game development, Freya makes some helpful videos that bridge the theoretical and practical divide.
Scott works at Microsoft and has an infections appetite to learn and discover. His Computer Stuff they Didn't Teach You playlist is filled with useful information that often falls through the cracks.