What's .idrc stand for?

Probably that's short for InDesign resource compiler.

There is no single .idrc, it is a bunch of binary formats determined by the folder containing the file.

The whole resources folder of nested folders is a replacement for a single MacOS Classic file's resource fork, each idrc_XXXX folder stands for one type, each numbered file stands for one resource.

Some files such as idrc_PNGA are just binary copies of .PNG .

Most other files are produced by the ODFRC resource compiler, whose ancestors are Apple's MPW Rez compiler, later the Apple+IBM OpenDoc resource compiler. Most idrc_ formats are described by matching sources in ".fh" files of the plugin SDK, and the responsible C++ routines use streams to load them from that binary format.

For example a randomly chosen plugin contains these resources:

idrc_CLST - class table describing the C++ object model

idrc_FACT - factory of implementation classes referenced by the class table

idrc_LOCR - localization indexes

idrc_PMST - localized strings

idrc_PVER - general plugin version info

idrc_VIEW - persisted views (dialogs, panels or other widgets)