The Code of Jewish Law (Shulchan Arukh) is based on the Mishna (and its commentary, the rest of the Talmud) but is not precisely identical to it. The Code of Jewish Law contains material in addition to the Mishna, since a lot of detail-specifying commentary has been written since then.
For the same reason that they wrote the mishna. People were forgetting the oral tradition so it had to be written down, now with more detail than the mishna. Also some Jews were living in Babylonia and some in Israel, the Jews in Babylonia would be teaching differently than the Jews in Israel. Therefore, they wrote the Gemara so that all Jews would have the same "guidelines"
The Mishna is 6 books that cotafie the Torah, The Gommorah has commentarys on the Mishna.
Orthodox Jews follow nearly all the laws. Conservative follow some, but not all. Reform Jews follow little or none of the laws.
The Torah.
The ten commandments
The people who follow Judaism are Jews.
People who follow Judaism are called Jews
they were administered by Jews and Jews had nominal control, but ultimately they had to follow Nazi decrees.
This is a semi-Hebrew phrase, but Jews would never use this phrase in prayer. Jews regard the name as too holy to pronounce (Talmud Sanhedrin, Mishna 11:1).
Jews first and foremost follow the laws of the land.
If you consider yourself Jewish but do not follow Judaism, holding to no religion at all, then you are a "secular Jew". A significant proportion of Jews are secular Jews. Some Jews have adopted a form of Christianity and consider themselves Christian Jews or Messianic Jews.
There's no exact number, but it's mostly Orthodox Jews who follow the dietary laws.