If a logical block address is not mapped to any physical flash memory addresses, then the SSD can return zeros for a read request immediately, without touching the flash.

Does the mapping happen on first write? Is TRIM then a command that signals the SSD to unmap that block?

Yes.

You read zeroes.

Not guaranteed by default for NVMe drives. There's an NVMe feature bit for "Read Zero After TRIM" which if set for a drive guarantees this behavior but many drives of interest (2024) do not set this.

Hmmm, when I quick-formatted a drive (which TRIMs the whole thing), then tried reading it back in a disk hex editor, I just saw zeroes.

It seems like that would be a likely common behavior for the FTL, but other options are possible (e.g., reading the old blocks) and it wasn't guaranteed by the spec, which is why they added this NVMe flag (so-called "DZAT") so that you can actually rely on it.