There is no "*.bs" file supplied with the extension. Instead, there may be a *_BS file which has code for the special cases, like posix for berkeley db on the NeXT.
This file will get parsed, and produce a maybe empty @DynaLoader::dl_resolve_using array for the current architecture. That will be extended by $BSLOADLIBS, which was computed by ExtUtils::Liblist::ext(). If this array still is empty, we do nothing, else we write a .bs file with an @DynaLoader::dl_resolve_using array.
The *_BS file can put some code into the generated "*.bs" file by placing it in $bscode. This is a handy 'escape' mechanism that may prove useful in complex situations.
If @DynaLoader::dl_resolve_using contains "-L*" or "-l*" entries then Mkbootstrap will automatically add a dl_findfile() call to the generated "*.bs" file.