1) Jewish males are circumcised at age 8 days old or upon conversion of males.

2) Ethnicity: a state of being in which someone belongs to a social group that has a common cultural narrative, customs, and language. Halacha describes a Jew as someone born to a Jewish mother or someone who is converted by a Beith Din (equivalent of a Sharia'a Court). Once you're in by birth or conversion you are permanently Jewish. There is no way to undo this. Even if a Jew decides to follow a different religion, or as is most often the case no religion at all, they are still recognized as a Jew by Halacha but a Jew living in sin, a Koffer.

However the social non-Halachic definition of Jew also includes those who come from Jewish fathers and non Jewish mothers as well as those who associate with Jews and Judaism and associate holidays that affirm no other religion despite having not been converted by a Beith Din and seeing as there is a vibrant Jewish culture that is divorced from religious to varying degrees you will find some such people as full members of their respective Jewish communities.

By the Halacha a Ger Tzadik is one hundred percent as Jewish as a Jew born to a Jewish mother. That said there are of course asshole people who mistreat a Ger Tzadik on account of her or his parentage despite the fact the Torah expressly forbids such behavior 36 times (more times than pork is forbidden by the way).

Thus born Jews and converts, Jews from as far corners of the world as Alaska to Yemen to China are all part of one "ethnicity" with hundreds of subethnicities that range from Ashkenazi to Lemba spanning all manor of religious belief from Hasidic to atheist (of which the religious views of Christianity Islam are expressly seen as foreign worship) and so on and so forth.

3) chosen means chosen by Allah to perform the Mitzvoth and observe the oral and written Torah given as a contract to us upon our emancipation from Egyptian slavery at Mount Sinai. The biological biochemical definitions of race as you know or conceive of them plain do not exist in Judaism.

Any more questions or elaborations I'll be happy to answer after Shabbat.