The Raspberry Pi 3B+ has an onboard Broadcom WiFi module that is closely integrated with the CPU cores of the PI. The primary interface between the two is an SDIO channel and the software support is buried deep in the Raspbian code. I am not aware of any workable SPI interface that will provide the same usability as the 3B+. I was looking at this development board for the compute module
https://www.amazon.com/gp/product/B07H8 ... _qh_dp_hza
and I think it was a shame they didn't include the Broadcom chip to provide both the Bluetooth and Wifi support that the Broadcom chip gives the 3B+.
In a way, it's too bad the compute module didn't get the Broadcom WiFi module as well, seeing that communication is so important. I hope someone will see that lack and provide the missing functionality. Also, it might have been nice to consider how to support a cluster of the compute modules with some common backchannel on the bus.