How are code segments and data segments of a source code program really handled and separated from each other during process execution?

I can explain how things work roughly on Windows.

First of all, the given information in the book does not apply that much to modern nowadays OSes. Most OS (such as Windows, Linux, etc.) has an executable file format that describes how the code and data are stored within the file, how they can be mapped into RAM, where to start to execute the code so on. On Windows, the format is called Portable Executable. PE format consists of zero or more sections to store the code and other data. Sections contain some important information such as how the OS will find the data of the section in the file, how to map this data to the memory, what kind of protection method will be used for this data in the memory. Sections can also have a name such as .text, .data, .bss, .idata, .rdata giving a clue about what kind of data the section contains.

When you compile and link your code with MSVC on Windows, you have a portable executable file for your program. This PE file will have one or more sections. For your example, it may have a .text for code, a .data for initialized data, and a .idata section for your imports from other modules. .text section has the compiled machine code, .data section has the data of value 10 for the variable test. When you execute the file, the OS loader will try to load, parse and map it into the memory created for its process.

So, you don't need a STORE instruction to store and initialize the data in RAM. All data in your program is located at the corresponding section and will be mapped into memory by the loader.