As a VSIX Extension:
If you install analyzers as a Visual Studio extension, they apply at the solution level, to all instances of Visual Studio.
You cannot set the severity of rules from analyzers that were installed as a Visual Studio extension.
Analyzer warnings and errors don't show up in the build report if you install the analyzers as an extension.
As a Nuget package:
If you install the analyzers as a NuGet package, which is the preferred method, they apply only to the project where the NuGet package was installed. In team environments, analyzers installed as NuGet packages are in scope for all developers that work on that project.
To have rules enforced at build time, including through the command line or as part of a continuous integration (CI) build, install the analyzers as a NuGet package.
We can configure rule severity, when analyzers are installed as a NuGet package.