Short answers, pulled from the story.
Mecha is an abbreviation of "mechanical," first shortened in Japanese. In Japanese, mecha encompasses all mechanical objects including cars, computers, and firearms, while the narrower term giant robot distinguishes limbed vehicles from other devices. Outside Japan, mecha refers specifically to large humanoid piloted machines.
Mazinger Z, written by Go Nagai and introduced in 1972, was the first anime to feature a giant mecha piloted by the protagonist from within a cockpit. Nagai designed the pilot to fly in via a small aircraft that docked inside the robot's head, introducing the idea of mecha as pilotable war machines rather than remote-controlled robots.
Japanese mecha designer Shoji Kawamori pioneered the transforming mech in the early 1980s. He created the Diaclone toy line in 1980 and the Macross anime franchise in 1982. His designs include the VF-1 Valkyrie and Optimus Prime, known in Japan as Convoy.
Edward S. Ellis's 1868 novel The Steam Man of the Prairies featured a steam-powered mechanical man piloted from behind, making it one of the earliest ancestors of the mech in modern fiction. Jules Verne followed in 1880 with a steam-powered mechanical elephant in The Steam House.
Prosthesis: The Anti-Robot, invented by Jonathan Tippett and unveiled by Canadian company Furrion Exo-Bionics in 2017, received the Guinness World Record as the world's largest tetrapod exoskeleton in 2020. It weighs 3,500 kilograms, produces 200 horsepower, and is controlled via a full-body exo-skeletal interface by the pilot inside.
Mecha are typically much larger than the person wearing or piloting them and are operated from a cockpit, usually located in the chest or head. Powered armor such as Iron Man's suit fits closely around the wearer. Iron Man's enemy the Iron Monger and the mobile suits of the Gundam franchise are cited as examples of true mecha.