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In olden days .net was Windows only. Mono was an independent effort to build a cross platform runtime. With .net core MS made it open source and cross platform.

In olden days .net was Windows only. Mono was an independent effort to build a cross platform runtime. With .net core MS made it open source and cross platform. They also bought Mono but that is more or less abandoned I believe latest versions of .net run well on Linux and Mac and now also in browser. They have also dropped Core from the name and there is only one .net which is open source and cross platform. Edit: seems some part of Mono still live.

WorldMaker 3y ago
Mono wasn't abandoned so much as "code merged" (a beauty from that both are now open source). Mono merged into the main .NET repo and the best parts of it became "a single runtime". There are still useful parts of Mono left that aren't exactly inside the "a single runtime" yet, but the amount of "Mono code" shrinks with each version and the more it is all "just" .NET.