What is the procedure to obtain source code for the functions within the STRINGdb package?

string_db <- STRINGdb$new( version="11", species=9606, score_threshold=0, input_directory="" )

I would like to understand the process behind the function string_db$map() and tweak it to change the functionality of the method.

I couldn't find the code in the Package Archive here: https://bioconductor.org/packages/release/bioc/bin/windows/contrib/4.0/STRINGdb_2.2.0.zip

I think you should be able to download all of the package's source code with git clone https://git.bioconductor.org/packages/STRINGdb . You will only be able to view the code and will not be able to push changes to git.bioconductor.org unless you are the maintainer with developer access.