Mapping between each bit in a bitmap index and tuple-pointers
First of all, there is no bitmap index type. The Bitmap Index Scan is just a fast mechanism to look up and sort values from a bitmap.

The Backend Process scans the Index for all tuples, and stores them in a structure called TIDBitmap. The pointers to the tuples are sorted so they could be accessed sequential on disk.

The source of the Bitmap structure could be found in src/backend/nodes/tidbitmap.c