Does signature-based Anti-virus software match each presently-scanned file with all of strings/signatures which exist within the database? Must it go through all signatures to be able to do a comparison having a file? The comparison is performed in the "database towards the file" not in the "file towards the database", is the fact that true ?

The 2nd question: Can you really extract the string/signature (NOT Hash for whole file) by Audio-video engine from the file first, after which decide if that string is incorporated in the database or otherwise? Can there be any known Audio-video does by doing this?

Typically how this is accomplished would be that the database of known virus signatures can be used to construct a condition machine (frequently something much like the Aho-Corasick string search algorithm). Then, each file that's to become checked operates with the condition machine. This works out to become remarkably fast, as all matching virus signatures are available having a single go through the file.