Where are the value types declare inside reference types are stored in memory in C#?

The "stack" and "heap" are implementation details, and not as important as the observable characteristics of value and reference types:
The Stack Is An Implementation Detail, Part One – Fabulous Adventures In Coding[^]
The Stack Is An Implementation Detail, Part Two – Fabulous Adventures In Coding[^]

But the terms can still be useful when thinking about how the types behave.

I think it would be more correct to say that the data for a value type is stored inline, whereas the data for a reference type is stored elsewhere (on the heap), with the address of that data being stored inline.

So, given a reference type containing two fields:

class Foo
{
    int x = 42;
    string s = "Bar";
}

and an instance of that type:

Foo f = new Foo();

f stores the address of the data for that instance - eg: 0x1234567. That data might look something like:

// i:
0x1234567:   0
0x1234568:   0
0x1234569:   0
0x1234570:  42

// Address of s:
0x1234571:  11
0x1234572: 173
0x1234573: 240
0x1234574:  13

The data for i is stored inline: 42. But the data for s is stored somewhere else, at address 0x0BADF00D:

0x0BADF009:   0 // Length
0x0BADF00A:   0 
0x0BADF00B:   0 
0x0BADF00C:   3 
0x0BADF00D:  66 // "B"
0x0BADF00E:  97 // "a"
0x0BADF00F: 114 // "r"

Of course, this ignores the concept of "boxed" value types[^], which are stored in the same way as reference types.

Some more light reading: :)
The Truth About Value Types – Fabulous Adventures In Coding[^]
Debunking another myth about value types – Fabulous Adventures In Coding[^]
Six important .NET concepts: Stack, heap, value types, reference types, boxing, and unboxing[^]