We all knows that .NET Core is platform independent and can run on any OS like Windows, Linux or Mac.
I am just little more curious to understand, how the Cross platform is achieved in case of .NET Core, where the same was not available in case of .NET Framework.
Would be helpful, if somebody explains.
Thanks in Advance
There are multiple factors contributing to that. important ones include:
The operating system provides many low-level functionalities from file access over networking to scheduling. Just as in C/C++, you may need to implement things differently for Windows than for Linux or macOS.
.NET Framework was windows-specific so many things had to be abstracted and implemented differently for Linux / macOS for compatiblity. E.g. using unix socket APIs for networking instead of Winsock.
Also note that Garbage Collection makes use of more advanced uses of memory and scheduling APIs which also don't exists 1:1 on all platforms so there are implementation differences.
Some features like Graphics (System.Drawing) or WCF hosting are acutally more a feature of Windows than of .NET Framework. Some of these had to either be reimplemented in pure .NET or separated out into support packages that you could use on .NET Core / .NET 5+ but would only work when run on windows. Others (WCF hosting) have been dropped altogether.
Another good example of this is globalization support, so handling language-specific details in .NET. This is used for many string operations, string formatting (e.g. formatting a month name in various languages) and so on. On Windows this used to use the National Language Support APIs but for Linux and macOS this was implemented for .NET Core using the International Components for Unicode (ICU) libraries. This causes some behavioral differences between these versions. In .NET 5+ even the Windows version of .NET will try to use ICU when available as it was included in Windows 10 in 2019.
.NET uses an intermediate language (IL) that user code compiles to that is not a machine executable format but can be translated into optimized for the native machine code of the system that the code is run on. This just-in-time compiler (JIT) needs to support all machine architectures (Intel archicture, ARM architecture, 32/64 bit, now WebAssembly is coming as well, ...). The JIT compiler in .NET Framework only supported the Intel instruction sets.
And some architectures / operating systems even have specifics for that. E.g. macOS 64 bit ARM (e.g. M1 chip) has some slight differences in calling conventions (needed for integration with the OS and libraries) than Linux. Furthermore the security system around running JIT compiled code requires some changes as well (write-XOR-execute memory pages).