Children learn about other religions and about Christianity. In all the state non c of e schools my kids have been to, and in the ones I went to, and in the ones my family members have taught in, they also pray in assemblies and sing songs about Jesus. Many have visiting “open the book” puppet shows from local churches. They learn about other religions in RE lessons, but the core running through so many of these schools is overtly Christian. I agree that there are some state schools who pull as far away from it as possible, and of course children can be opted out of attending assemblies etc. but it’s quite othering and most won’t.
At secondaries it appears to be much less pronounced.
Surely you’ve seen all the threads on here about Christianity in state schools? It’s hardly news. Very normal for kids to come back from school talking about the baby Jesus and to be introduced to Christianity as an assumed fact. Maybe as a Christian yourself you don’t spot it as easily as it feels “normal” to you. Maybe you have worked mostly in secondary schools. Maybe you worked in a school where the head worked hard to keep a good balance and to move as much away as possible from the required “broadly Christian nature”.