What exactly is in a .o / .a / .so file?

I was wondering what exactly is stored in a .o or a .so file that results from compiling a C++ program. This post gives a quite good overview of the compilation process and the function of a .o file in it, and as far as I understand from this post, .a and .so files are just multiple .o files merged into a single file that is linked in a static (.a) or dynamic (.so) way.

But I wanted to check if I understand correctly what is stored in such a file. After compiling the following code

void f();
void f2(int);

const int X = 25;

void g() {
  f();
  f2(X);
}

void h() {
  g();
}

I would expect to find the following items in the .o file:

• Machine code for g(), containing some placeholder addresses where f() and f2(int) are called.
• Machine code for h(), with no placeholders
• Machine code for X, which would be just the number 25
• Some kind of table that specifies at which addresses in the file the symbols g(), h() and X can be found
• Another table that specifies which placeholders were used to refer to the undefined symbols f() and f2(int), which have to be resolved during linking.

Then a program like nm would list all the symbol names from both tables.

I suppose that the compiler could optimize the call f2(X) by calling f2(25) instead, but it would still need to keep the symbol X in the .o file since there is no way to know if it will be used from a different .o file.

Would that be about correct? Is it the same for .a and .so files?

Thanks for your help!