My question is it is another version of powershell or module or tool or what?
According to official documentation:
A .NET tool is a special NuGet package that contains a console application.
This applies to PowerShell. dotnet tool, like python -m pip and myriad alternative examples across programming languages' official implementations, is the official package manager for DotNet (.NET).
Consequently, the aforementioned command installs PowerShell in a manner that your OS's package manager doesn't understand, but which is standard amongst .NET packages.
And another question is that without this Powershell(dotnet global) – Will I not be able to install any module?
It supports modules installed via Install-Module.