1) You don't have to buy Monogame. Monogame is downloaded from the Internet for free and is installed free of charge, because it is free development (Open-Source).
Monogame the project is created in Visual Studio, which is also downloaded for free Visual Studio Community (For non-commercial use, students, open source developers)

2) The language C# and .NET of the company's technology Microsoft are used. It is a modern, fast programming language.

3) Cross-Platform. The code written in the Monogame project is compiled and run on all platforms: iOS, Android, MacOS, Linux, Windows, PS4, PSVita, Xbox One, Nintendo Switch.

4) There are communities that are constantly updating and adding new features to Monogame and there are communities of programmers, developers who write games in Monogame. Official website Monogame: http://www.monogame.net