in Mac OS 8 and 9, you could patch the system direct. OS X was harder, and became sprawling and interdependent, encouraging users to reinstall macOS. That has now ended with the introduction of the SSV.
TomeViewer
In comparison with OS X, Mac OS 8 and 9 were tiny and simple. Installing or replacing a component was a minor task; here is how we did it.
The Eclectic Light Company 