Official Erlang documentation on this topic
term_to_binary/2 docs
C implementation of the term_to_binary in ERTS.
Garbage collect C implementation.
erl_create_process function.
Be prepared to wait a liitle bit because erl_process.c file has almost 13500 lines of C code. This is a cornerstone of the whole Erlang platform I think.
We always use BIFs in Erlang such as element or spawn_link or whatnot in our code.
gen and gen_server are based on the erlang:spawn BIF, hence I think it's important to know how BIFs are actually implemented.
https://github.com/erlang/otp/blob/master/lib/stdlib/src/erl_internal.erl#L22
stdlib/erl_internal here is just a BIF-proxy because all the functions are implemented natively in C inside the ERTS/BEAM.
Core C implementation of BIFs resides here
The most important parts from the Erlang code purger such as purge, soft_purge, etc.
Lists module functionality. Some of the functions such as keyfind/3, keymember/3, etc. are implemented as BIFs and defined in a separate C file inside of BEAM.
Core functionality of the Erlang message passing
https://github.com/erlang/otp/tree/master/erts/emulator/internal_doc