Auto-loaded type information and relationship between .til and .sig: how to find links and how to change them (config)?

Question

NB: most of my targets are PE files for pc, so I'll stick with that for the question. But I suppose the principle holds also for the related directories, like $IDADIR/sig/arm.

Let's say I load a PE file, which happens to be a 64-bit Windows driver. I may then see in the Output pane (redacted for brevity):

IDA is analysing the input file...
You may start to explore the input file right now.
...
Using FLIRT signature: Windows Driver Kit 7/10 64bit
Propagating type information...

Now "Windows Driver Kit 7/10 64bit" corresponds to $IDADIR/sig/pc/ms64wdk.sig (found with dumpsig). Fair enough. I also know that $IDADIR/sig/pc/autoload.cfg governs what type information library (.til) file gets loaded based on the loading of a signature.

Alas, there is no entry that maps the aforementioned $IDADIR/sig/pc/ms64wdk.sig to ntddk64_win7 and ntapi64_win7. Yet, when I use Produce file -> Dump database to IDC file... the resulting .idc file has the following calls among its first actions (again redacted for brevity):

#define UNLOADED_FILE   1
#include <idc.idc>

static main(void) {
    // ...
    GenInfo();            // various settings
    // ...
}

static GenInfo(void) {

    // ...
    add_default_til("ntddk64_win7");
    add_default_til("ntapi64_win7");
}

Questions:

• How does IDA make the connection between the FLIRT (.sig) and the .til here? (Apparently $IDADIR/sig/pc/autoload.cfg has no bearing.)
• Given there are also other .til files like ntddk64_win10.til, ntapi64_win10.til:
  • How can I configure IDA to load those instead of the somehow implicit ntddk64_win7 and ntapi64_win7 respectively?
  • Assuming I can't override the behavior or have an existing IDB which had the implicit ntddk64_win7 and ntapi64_win7 applied: If I now use add_default_til("ntddk64_win10") and add_default_til("ntapi64_win10") (no matter whether IDC or IDAPython) will this adversely affect existing usage of type information (e.g. enums which were imported from standard enums)? How does it affect existing usage?
• What is the relation between the ntddk64.til and, say, ntddk64_vista.til, ntddk64_win7.til, ntddk64_win8.til, ntddk64_win81.til and ntddk64_win10.til? Is ntddk64.til the superset? Some subset?

PS: Pointers to official documentation very much welcome.