What is this?
This is dotnet/corefx, the GitHub home of the .NET Core base class libraries (CoreFX). The repo is archived and now serves only as the place for servicing pull requests for .NET Core 2.1 and 3.1.
So is this something I install and run, or a library I add to my project?
Neither: it is the source repository of the .NET Core base class libraries, which ship as part of .NET Core itself. You use them through .NET Core, not by pulling code from this repo.
Is this repo still active?
No. It is archived: the .NET team develops the code and handles the issues in dotnet/runtime, and the only work accepted here is servicing PRs for .NET Core 2.1 and 3.1.
Who would still come to this repo today?
Mostly .NET maintainers backporting fixes to .NET Core 2.1 and 3.1. Developers working on current .NET versions should go to dotnet/runtime instead.
Where did this project go after the consolidation?
To dotnet/runtime. The .NET team now develops the code and tracks the issues formerly filed here there; the consolidation itself is described in dotnet/announcements#119 ('Consolidating .NET GitHub repos').
What are similar projects?
coreclr, awesome-dotnet-core, efcore, awesome-dotnet, eShopOnWeb, server, csharplang, aspnetboilerplate
Which topics is it in?
dotnetcore, dotnet-core, microsoft, dotnet, programming, c#, csharp, deprecated, developers, framework, api, library