What is the real day for rest? (Shabbat)

So, I was searching online about the rest day because in the Bible the days are only mentioned like this, day 1, day 2 etc

I’ve read online that Jewish people hold the Shabbat on the 7th day which is Saturday, not Sunday and I came here to ask if this is real.

Another question would be that, in which day are Jewish people going to church? Is it in the same day as Shabbat?

Please be patient with me and I’m sorry if I did not write everything correctly or if I offended someone with my questions.