to belong to judaism are you converting as a member of a nation?

I am not sure if judaism is a race or not but I don't get why do jewish people who were born a jewish mother are automocally jew even they don't practice the judaism.. even they change to another religion.. they will be considered from a jewish heritage? it was judaism a religion of the hebrews and now after the destruction of the second temple it became open to other people but you have to adopt the rules of the judaism as it were a spiritual nation or race?
I hope you understand my question.. I mean when you are accepting judaism.. you are converting as a member of a spiritual nation also part of israel because if you want to live there you have chances
well when a jewish perso wants to emigrate to israel, it has free pass than a non-jewish person. it's called aaliyah or something like that
sorry for my bad english
 
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