How this script works

1. Takes your module specifications and builds a declarative dependency tree, following a IaC plan/apply approach to installing modules. By default it will evaluate locally installed modules in all of your configured PSModulePath to determine if an already-installed module meets the criteria.
2. Uses a custom HTTPClient and async tasks to build the dependency tree quickly, querying with minimal data possible and using batch queries where possible to optimize for performance. This is all done in native PowerShell, no C# required (though C# would allow for certain operations to be even more parallelizable by chaining tasks).
3. Uses HTTP/2 to run all queries in a single TCP conversation where possible (PSGallery supports it)
4. A custom nuget v3 mirror of PowerShell Gallery called pwsh.gallery uses Sleet to mirror all PowerShell Gallery packages and is fronted by Cloudflare for high performance, high availability, and data locality.