If you already have some developer background, I suggest not to take any courses.

When I have an idea and I want to learn a new language or framework, I go and try to implement that idea in the new environment. One example could be the previous version of my blog. It was a (bloated) Symfony app; I wanted to learn Python. So I rewrote my blog in Python using Django. Today Python is my $job.

If you already have an app idea, go and write it. Do a very beginner course only to learn how to set up a development environment, how to compile your project, and how to debug your app while running it in an emulator or on your phone. Then create small tasks to implement, and by the time the app is ready for a beta release, you already know Android development (to some extent).

This is a very common question to developers who want to learn a new language or newbies who are just starting out in their programming journey. For me, I would advice you understand the approach you want to take. There several ways to develop android apps: native, hybrid, or even mobile web. All these approaches have their different development environments and tools you have to set up. They all have their individual learning curves, especially for newbies. But hybrid apps may be easier for someone who is already a developer on the web stack. Personally, I prefer the native route and I do a lot of native android development.

So after getting a grasp of what approach and tool you want to employ for your development, then take a small project and work on. Overtime I have observed that just watching online courses without having a project to work on is a mere waist of time. Its's best to learn by doing. So pick a small project or one you have done in another language and rewrite it in android. Download the necessary development environment, setup and go through the 'getting started' section on android development website. You are good to go.

Google, in collaboration with Udemy, has developed great courses for beginners, developers, and even advanced android courses that are free online. Please see the link below.

developers.google.com/training/android

I'm not sure there's a "best" way to learn or even best course. It all depends on how you learn. Personally, I learn by doing. That means, install Android Studio, then run through a tutorial or two just to get a feel for it. After that, I take my idea and start implementing. For me, I usually think about what it is I want to do. For example, "I want to have a menu in the top right corner". Then, I go and look for specific information about how to accomplish that particular task. Eventually, it all starts coming together. That may not be the most efficient way of doing things, but it works for me.

One reason I do not recommend a particular course is because you never know how outdated it might be. Unless the course specifies, for all you know, the course could be covering Android development from two or three years ago. That wouldn't be good to learn. You want something more up-to-date.

I would say this: If you don't know Java or Kotlin, you may want to learn a bit about those, as Java is the native language for Android, and Kotlin is quickly becoming adopted. If you're interested in more cross-platform development, there are quite a few options for writing in Javascript and compiling to native code.

I'm not personally an Android or iOS developer, but I've done a little in both, writing my own app for personal use. Since I'm mainly a Windows user (because my job requires it), I find that Android development on Windows is painfully slow, so I have stayed away from it myself. However, the little bit I learned was by following the advice above. Fortunately, I know Java, so that was the easy part.