As far as I know, .NET is just a framework but it had a problem, so Microsoft came up with .NET Core to make it cross-platform compatible(works on different OS). Then they stopped updating .NET and only worked on .NET Core. Then they realized why use the naming ".NET Core" when you have an empty name ".NET" because remember they stopped updating .NET so it was gone. That's why after version 5 or something, .NET Core was renamed to .NET. So, today we use .NET, which is actually .NET Core. Old .NET is gone.

When it comes to .NET standard, it was used to link old .NET with .NET Core in old times but once they only had .NET(new .NET Core), there was no need for .NET standard. This is because old .NET is gone, so no need to link anything. There was only one thing left - .NET which was actually once upon a time .NET Core.