For AutoCAD 2021 release we have posted API package on to Nuget.
We have started posting AutoCAD .NET packages from AutoCAD 2015 release, from since then we regularly update and post .NET package as per AutoCAD release cycle.
You simply select Project –> Manage NuGet Packages in your Visual Studio project:
Search for AutoCAD .NET click install on the primary AutoCAD.NET package (the third in the list). This will install acmgd.dll and its related assemblies but also the other two dependent packages. You should install AutoCAD.NET.Core (i.e. acmgdcore.dll, etc.) if you want to create a Core Console-compatible project, of course. Either way AutoCAD.NET.Model (i.e. acdbmgd.dll, etc.) will get installed.
Further reading on .NET package can be found on Kean’s old post.