using a shared library : custom myfopen(), myfrwrite()

This may require some trial and error. I see two ways to do this. One is to run potential commands that deal with files in bash and see if when traced they call fopen:

strace bash -c "read a < /dev/null"`

or

strace bash -c "read a < /dev/null"` 2&>1 | fgrep fopen

This shows that that read uses open, not fopen.

Another way is to grep through the source code of bash as @oguz suggested. When I did this I found several places where fopen is called, but I did not investigate further:

curl https://mirrors.tripadvisor.com/gnu/bash/bash-5.1-rc3.tar.gz|tar -z -x --to-stdout --wildcards \*.c | fgrep fopen

You'll want to unarchive the whole package and search through the .c files one by one, e.g.:

curl https://mirrors.tripadvisor.com/gnu/bash/bash-5.1-rc3.tar.gz|tar -z -v -x --wildcards \*.c

You can also FTP the file or save it via a browser and do the tar standalone if you don't have curl (wget will also work).

Hopefully you can trace the relevant commands but it might not be easy.