I spent a little time looking into it, and it turns out that Microsoft owns an outfit named GitHub, which apparently offers free hosting for open source projects and is used by a lot of open source software makers. And Microsoft has around 4k employees with contributions to OS projects on GitHub. So maybe "biggest" is a lie in light of their anti-OS efforts, but it is at least arguable. Can you propose a bigger one?
Microsoft is the biggest supporter of Microsoft gaining market share.
You mean the outfit that has misappropriated the code of those hosted projects to train their commercial code generator which is a very clear violation of the spirit if not the letter of the licenses of many of those projects?