You can see an illustration of the RP3A0 system-in-package in the official blog post[1].
From what I can gather[2] the spacer is actually an interposer, which for all intents and purposes functions as a PCB between the two chips, allowing for signals to be routed between the chips. Think of it as two IC's on either side of a PCB with vias connecting them. No expert though so maybe I'm mistaken.
I was assuming they called it an interposer for a reason. However as the sibling comment by mkoc mentions it might just be a spacer, and the X-ray image[1] of the package seems to confirm that.
From what I can gather[2] the spacer is actually an interposer, which for all intents and purposes functions as a PCB between the two chips, allowing for signals to be routed between the chips. Think of it as two IC's on either side of a PCB with vias connecting them. No expert though so maybe I'm mistaken.
[1]: https://www.raspberrypi.com/news/new-raspberry-pi-zero-2-w-2...
[2]: https://sst.semiconductor-digest.com/2011/06/silicon-interpo... (section "Alternative 3D technology")