When compiling multiple source files

[ProgrammingGhost]

How does the compiler do static typing of multiple source files?
I heard D malloc memory and doesn't free to speed up compilation
but I am guessing every instance doesn't compile just one source
file? My question is if I have a function in this file and
another in a different file what does the compiler do when both
files needs to know the definition of another? Also how does it
handle modules?

   From another thing I heard text parsing can be ridiculously fast
so there may be no need for a binary representation of each file
parsed. Does the D compiler read all source files into memory
generate the AST then starts compiling each file? I know there
more than one compiler but I wouldn't mind hearing from either or
both if they differ.