Like CalMeacham said, they vary quite a lot - not only between churches, but different services within the same UU church may be very different.
I’ve been at some that were Generic Christian - that is, they used a hymnal and New Testament, but the words “Jesus” and “Christ” weren’t actually mentioned that I recall; “The Creator” and “God” were used.
In another service, the officiant talked about Faith, and about doing good things for yourself and others, but it was pretty agnostic. She even mentioned something to the effect of “Whatever god or gods you may believe or not believe in”, which drew a chuckle from the congregation, and then talked about how you can hold Faith in something you don’t entirely know if you believe in. Very thought provoking sermon.
In a service I attended once that was run by a practitioner of Feri Wicca, it was seats in a circle and an interesting and unorthodox (by Wiccan standards) calling of the elements by using the symbols within the church. I remember that Fire was the UU flame; I don’t recall the others. She then had us pass around a bowl of beans and take a few, then invited us to ask our neighbors for some of their beans. We could say yes, or no, give them more beans than they asked for, or fewer, or ask them for beans in return. When that was done, she talked about how the beans were an example of how we (each as individuals, but also as a group) share resources, like money and time, and that if they don’t circulate, they don’t do anyone any good. She didn’t call anyone out, but you could see people who had refused to share squirming in their seats just a little bit, and the kids who had decided to pool their beans into a big pile they all “owned” got very thoughtful as they realized they had to figure out how to negotiate group ownership. I sat there with empty hands and realized that if I give everything away, I might be liked, but I have nothing to sustain myself. Another very thought provoking morning, with no overt mention of religion at all.
I think my favorite evening at a UU church was a combined Christmas/Winter Solstice, with a fairly standard Christian Christmas service inside, including a Nativity Play and choral singing, while outside I sat with the Neopagan members of the church, tending a Midwinter fire with the Yule Log - the stump of the previous year’s church Christmas tree. When the Christmas service was over, many of the people who had been inside came outside, and we sang some songs - some pagan, some Christian, and some secular - and shared warm cider and hot chocolate. That night, to me, was the very best of the best.
My apologies if I caused offense.