the Quantum Mechanical Keyboard firmware is a fork of the TMK firmware made by Hasu. It's got a lot of new features and compatibility with a lot more keyboards. It makes the firmware very programmable. VIA builds upon that with a standalone GUI, and features lots of many new add-ons like plug and play (no flashing), RGB controls, switch tester, etc.

QMK and VIA are two separate packages however VIA is built on QMK. Watch some MechMerlin videos on YouTube - he explains them very well.

Reddit source

QMK

Bindings & keycodes basic | github oos