As of commit 7b8969ce5cb2edc54f2c1aa39a85a3a08076337d on markdown-it master, the relevant source file is lib/rules_core/replacements.js and the relevant test fixture is test/fixtures/markdown-it/typographer.txt.
All of the replacements are hard-coded. They include (c) → ©, (tm) → ™, (r) → ®, and (p) → §, which are grouped as “scoped abbreviations”.
For what it’s worth, I don’t get (p) for §. That should almost certainly be ℗, the phonorecord symbol, instead.
I am going to poke at this for a few and see if I can’t come up with a patch that makes the “typographer” feature more configurable.