defining default folder?

this is confusing me..

firstly, if i name a file to open in a buffer~ objects arguments, wher does it look for that file? what happens if there are two identical files in different folders in the search path?

second, when i send a write message to buffer~ where does it save my file to? i know its maybe the default folder? but i'm not having much luck...tried filepath object, doesnt seem to do much