"Internals" is a poor choice of term. "Data structure" is a better term. Git is "plumbing and porcelain". The plumbing is the core of git. Porcelain are shortcuts. In general, Torvalds projects (Linux, Git) aren't big on abstractions that maximize simplicity-of-use, they focus on doing complex things correctly and quickly. Adding abstraction makes it hard to get details correct and run quickly.