I am confused. I tried numerous times. How this will lead me to qubes-core-qrexec/doc/multifile-policy.markdown at master · QubesOS/qubes-core-qrexec · GitHub exactly?

No - you said you could not find it with Google either.
yes, since I couldn’t find it via Github search.

You need to work on your search-fu.
I’m trying, first exploring myself, then asking for help, but can’t get simple answer how to reach qubes-core-qrexec/doc/multifile-policy.markdown at master · QubesOS/qubes-core-qrexec · GitHub

user:qubesos extension:.markdown filename:multifile-policy language:Markdown
If I use this, I get

Build software better, together
GitHub is where people build software. More than 100 million people use GitHub to discover, fork, and contribute to over 420 million projects.
Not to say that if I use this on Google, what meaningless results I get

Learn how to use the tools.
I’m trying, first exploring myself, then asking for help, but can’t get simple answer how to reach qubes-core-qrexec/doc/multifile-policy.markdown at master · QubesOS/qubes-core-qrexec · GitHub via Github search
And agian:

user:qubesos extension:.markdown filename:multifile-policy language:Markdown
when I manually type it, or choose it via Advanced search, I’m getting
We couldn’t find any repositories matching ‘user:qubesos extension:.markdown filename:multifile-policy language:Markdown’
You could search all of GitHub or try an advanced search.
If i click on Repository search results · GitHub obviously as you can see I’m getting first to sign in again. Clicking on Avanced search is meaningless, since I came there via Advanced search.
So… where I’m wrong? No, really…