AWS hasn't published all that much about their network, but we know they use a high-radix folded Clos fabric because they mention it in their paper about Scalable Reliable Datagram. If you want an overview of the folded Clos fabric, try reading Google's "Jupiter Rising" paper. https://dl.acm.org/doi/pdf/10.1145/2829988.2787508

Then, if you want to know about using optical switches to connect Clos segments without a fixed spine, check out Google's "Mission Apollo" paper: https://arxiv.org/pdf/2208.10041