Several parts of the repository (LevelDB, crc32c, secp256k1 etc.) are subtrees of software maintained elsewhere.
Some of these are maintained by active developers of Bitcoin Core, in which case changes should go directly upstream without being PRed directly against the project. They will be merged back in the next subtree merge.
Others are external projects without a tight relationship with our project.
There is a tool in test/lint/git-subtree-check.sh to check a subtree directory for consistency with its upstream repository.
See the full subtrees documentation for more information.
Many of the classes found throughout the codebase use the PIMPL technique to separate their implementation from the external representation. See PIMPL technique in the Appendix for more information.