Introduction

Xamarin.Forms code runs on multiple platforms - each of which has its own filesystem. This means that reading and writing files is most easily done using the native file APIs on each platform. Alternatively, embedded resources are a simpler solution to distribute data files with an app.
Prerequisites
  • Visual Studio 2017 or Later (Windows or Mac)

Setting up a Xamarin.Forms Project

Start by creating a new Xamarin.Forms project. You’ll learn more by going through the steps yourself.
Visual Studio 2019 has more options in the opening window. Clone or check out the code from any repository or, open a project or solution for your computer.
Now, you need to click "Create a new project".
Xamarin.Forms - Add Android Project in Existing Project
Now, filter by Project Type: Mobile
Choose the Mobile App (Xamarin. forms) project under C# and Mobile.
Name your app. You probably want your project and solution to use the same name as your app. Put it on your preferred location for projects and click "Create".
Now, select the blank app and target platforms - iOS and Windows (UWP).
Subsequently, go to the solution. In there, you get all the files and sources of your project (.NET Standard). Now, select the XAML page and double-click to open the MainPage.Xaml page.
Now, here there are only two platforms (iOS, and UWP).
Xamarin.Forms - Add Android Project in Existing Project

Add Android Project

First, you need to create a new Xamarin.Forms project with Android Platform. There is another way you can directly add an Android project to your existing Xamarin.Forms project. But I found this the best way.
Xamarin.Forms - Add Android Project in Existing Project
Now, go to your newly created Xamarin.Forms source. See, here is the Android project. You can copy it.
Xamarin.Forms - Add Android Project in Existing Project
In this step, go to your existing Xamarin.Forms project. Here, we are not having an Android project. So, paste here your copied Android project.
Xamarin.Forms - Add Android Project in Existing Project
Now, add the existing Android project to your solution. Alright, your Android project is almost ready.
Xamarin.Forms - Add Android Project in Existing Project

NuGet Packages

In this step, check your Xamarin.Forms version in NuGet. Make sure both platforms are in the same version. If not same please update your new Android project xamarin.forms package.
Xamarin.Forms - Add Android Project in Existing Project

Add Reference

Now, add your .NET standard reference to your newly added Android project.
Xamarin.Forms - Add Android Project in Existing Project
Everything is done.
Click the Play button to try it out on your Android device or emulator.
Xamarin.Forms - Add Android Project in Existing Project
I hope you have understood how to add Android project to an existing project in Xamarin.Forms. Thanks for reading. Please share your comments and feedback.
Happy Coding :)