I didn't followed the Youtube comments, however the comments from the Twitch chat are strongly biased to the no-OOP camp ... It is interesting to see how Casey's followers are more vocal than him. While Casey has technical arguments to reject OOP (more or less justifiable) the disciples seem to have (at least some of them) a visceral hate for OOP which I find hard to understand from such inexperienced people (most of them are students).
I've had a completely different experience. Phaser is one of the easiest to use JavaScript game engines and the documentation is well written, also they have plenty of examples. If you prefer to learn from a book, there are also a few books available.
Lets try a metal experiment, you have your knife and 3 enemies attack you at the same time. How many can you neutralize from 5 feet with your knife ? Suppose you have special training and you can throw your knife and kill one of them, you still have 2 ... Try the same mental experiment when you have a firearm in your hand ...
You can inflict much more damage with a firearm even without special training (not everybody can throw a knife and do some damages).
Pretty sure if you make more than 1mil per year you can afford to buy VS. The community edition is intended for indies, hobbyists and small enterprises.
There is no pre-made recipe. You define the scope of the project, you split it in a few "modules" with a well defined interface (functions headers in this context) and data structures and you start coding.
You should do some searches to check how big C projects like the Linux kernel (or any of the BSDs kernels) is managed.
gfortran which is part of GCC supports modules (this is because Fortran the language supports modules).
If you meant modules for C or C++, no (as far as I know), probably because C and C++ have no standard for modules. In a recent talk Bjarne Stroustroup talks about his hope that C++17 will include support for modules https://www.youtube.com/watch?v=2egL4y_VpYg
If you are on a Mac, you should definitely give a try to XCode it is, after all, the official Apple IDE. Not even close to Visual Studio, but not as bad (for C++ at least) as many people advertise.
For Linux, Kdevelop seems nice, other Linux C++ IDEs: Eclipse, NetBeans, CLion.
You should also probably try Vim and Emacs at least once, start with their included tutorials.
Because you can't use .NET CoreCLR as it is on Android. You will need to modify it in order to be able to compile it for Android. Here is a discussion about an attempt to modify .NET CoreCLR in order to be able to compile it for Android https://github.com/dotnet/coreclr/issues/1097
On iOS the situation is even worse you will need to have an AOT compiler for C# in order to be able to ship iOS apps in the Apple Store.
You are making a confusion here. Unity doesn't use Xamarin at all, they have an old license (like really old, since before Xamarin even existed) for Mono for iOS and Android.