Not your father's Microsoft
Microsoft's contribution to open source are numerous. They rank in the top 1 or 2 in most surveys.
"Microsoft came out on top in terms of total contributors (about 1,300), compared to second-place Google (about 900 contributors), while Google topped the charts in terms of actual code pushed to repositories (about 1,100 compared to Microsoft’s roughly 825)."
SONiC was built on Linux, runs Azure network control plane, and is made available for the community to use.
Disclaimer: I work for Microsoft and contribute code to the open source community. I also volunteer teach Linux hands-on workshops to kids 11-18.