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I don't believe this is entirely true anymore. Yes, years ago, the baseband processor (and firmware) had full DMA capabilities to the RAM ostensibly managed by the OS, and could do nefarious things if it wanted to. But I believe nowadays the baseband is a bit more isolated, and communication with it is mediated by the CPU and OS.

Some manufacturers likely still implement the "old" architecture, though.

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You would hope but to a large extent SMMUs are still not meaningfully deployed in consumer smartphones. Even when they do exist, the bounding enforced upon them are so expansive that it's essentially pointless. For example, one device I found had an SMMU in front of its BT/WiFi chip but, unfortunately, the driver on the AP side configured the SMMU to have access to all of system memory. Baffling.

How can I find out what peripherals my smartphone's baseband processor has access to?

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