Tiny Code Generator (TCG)
The Tiny Code Generator (TCG) exists to transform target insns (the processor being emulated) via the TCG frontend to TCG ops which are then transformed into host insns (the processor executing QEMU itself) via the TCG backend.
People who wish to port QEMU to run on a new processor need to be concerned with the backend. There also exists the TCI (TCG Interpreter) effort which provides a backend agnostic interpreter for TCGops.
People who wish to port QEMU to emulate a new processor need to be concerned with the frontend.