Short answers, pulled from the story.
The credit is disputed. Historian Joseph W. Roe credited Eli Whitney between 1912 and 1916, but later scholars, citing Edward A. Battison's 1966 article in the Smithsonian Journal of History, concluded there is no evidence Whitney developed or used a true milling machine. Battison suggested the first true milling machine was built by Robert Johnson of Middletown, Connecticut.
In face milling, cutting occurs primarily at the end corners of the cutter and is used to produce flat surfaces or flat-bottomed cavities. In peripheral milling, cutting occurs along the circumference of the cutter, shaping the cross section of the milled surface to match the cutter's profile; this method is suited for cutting deep slots, threads, and gear teeth.
Rudolph Bannow conceived the Bridgeport milling machine in 1936, and his company began manufacturing it in 1938. By the 1980s an estimated quarter-million Bridgeport machines had been built. The name became virtually synonymous with the category of manual milling machine, and the design is still in production.
The first NC machine tool was a Cincinnati Hydrotel milling machine retrofitted with a scratch-built NC control unit, reaching the developmental stage of laboratory reality in 1952. It was reported in Scientific American.
CAT tooling was invented by Caterpillar Inc. of Peoria, Illinois, and is the oldest and most common standard in the United States; it uses an imperial thread for the pull stud. BT tooling uses the same NMTB body taper but is symmetrical about the spindle axis, giving it greater stability at high speeds, and it uses a metric pull stud thread.
Gang milling uses two or more milling cutters mounted on the same arbor in a horizontal setup, allowing multiple features such as a slot, a flat surface, and an angular groove to be cut simultaneously. It was especially important before the CNC era because it reduced machining time for duplicate parts; CNC mills with automatic tool changers have largely replaced it.