Short answers, pulled from the story.
German forces built purpose-made devices like the Flachmine 17 by the end of World War I in 1918. These early devices were wooden boxes packed with explosives triggered remotely or by pressure.
The Tellermine 29 entered service in 1929 as a disc-shaped device approximately 30 cm across. It held about 5 kg of high explosives inside an outer casing that served only as a container.
Combat engineers laid 503,663 AT mines during the Battle of Kursk on the Eastern Front. This achievement created a density of 1500 mines per kilometer which was four times greater than what was seen in the Battle of Moscow.
Engineers now use non-ferrous materials making them harder to detect by metal detectors. Most modern mine bodies are made of plastic material to avoid easy detection while featuring combinations of pressure or magnetically activated detonators.
The Soviet Union began developing mines in the early 1920s. In 1924 engineers Yegorov and Zelinskiy produced the EZ mine with a 1 kg charge capable of breaking tank tracks.