The rules on what constitute kosher food is quite facinating - and complicated! Wikipedia is probably a better source than me writing as to what I can remember:
https://en.wikipedia.org/wiki/Kosher_foods
And things that are restricted on Shabbat are equally interesting:
https://en.wikipedia.org/wiki/Activi...ted_on_Shabbat
I understand that some orthodox Jews, who are not permitted to turn on the electric on Shabbat, have a workaround in putting on a slow cooker overnight or have a non-orthodox Jew or Gentile come over and turn the cooker on. And it was fascinating on an Easyjet flight to Israel last year to see the women in the plane, most of whom who were Orthodox Jews, taking wigs off for the duration of the flight and only putting them back on over their natural hair on the descent to Tel Aviv.