.NET (standard, Core and Framework): cross-platform application development

Question

I searched for SO, but I find many answers, but I do not aim my questions:

AS definitions say:

• The .NET Framework 4.6 is a complete framework for everything related to .NET, but it lacks libraries and runtimes optimized for multi-platform (Windows, Linux, and Mac) and cloud deployments.
• .NET Core, on the other hand, is a subset of the .NET Framework 4.6, which is mostly optimized for multi-platform and cloud deployments.
• NET standard: A standard set of APIs, not a platform. So basically you don’t need platforms, just a standard (version 1.3, 1.6 ...), and your code will work on all platforms supporting it.

To develop cross-platform applications, I intend to create core ASP.NET (.NET) applications that can reference other class libraries that can use:

• .NET standard
• .NET Core

I assume that if I use any (standard or basic), my application will still support another OS.

In some cases, I see people using the traditional .NET Framework with the .NET Standard in the class library?

• In this case, if the application is still running on the platform when using the .NET Framework?
• When should I mix and use a combination (Standard, Core, NET)?
• What are the best practices for using and mixing between frameworks?
• How to avoid conflicts and build failures?

For example, let's say I have a class library that has project.json:

 "dependencies": {
    "Microsoft.Extensions.Caching.Abstractions": "1.0.0",
    "Microsoft.Extensions.Options": "1.0.0",
    "StackExchange.Redis.StrongName": "1.1.608",
    "NETStandard.Library": "1.6.0"
  },
  "frameworks": {
    "netstandard1.5": { }
  }

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Answer 1

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EDIT:

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