What would be in out.txt?
cat < in.txt > out.txt
It would be the contents of in.txt.
Nothing. The redirect will create a new empty file but there will not be any output from the cat command to redirect.
Nothing because you can't redirect from file (in.txt) to another file (out.txt). You can only redirect from a command to a file.
The output from the command line. By default STDIN comes from the keyboard.