The code stores “digits” the size of a machine word, and keeps the count for those in the upper bits of the header.

Some good places to read in the OTP source:

• otp/erts/emulator/beam/erl_term.h: a nice diagram of how tags are used in a term’s header. A bignum’s sign is stored in the tag.
• otp/erts/emulator/beam/big.h: some C macros that access different parts of a bignum:
  • the sign is in the lowest bit of the header
  • the digits all immediately follow the header (the +1 in BIG_V)
  • the arity (number of digits) is stored in the header
• otp/erts/emulator/beam/big.c: the implementation of most bignum functions. The link goes to I_comp which tells us about how digits are stored
  • the first two cases dispose of the easy parts, when one number is more digits than the other
  • the final case does the digit-by-digit comparison, starting from the digit farthest from the header as indicated by setting up the loop with x += (xl-1)

One final thing - normally I’d point people towards the BEAM Book which has an epic amount of internal detail, but alas the section on bignum representation is marked TODO