Several manufacturers have "relay modules" that switch a relay when the smoke alarm sounds (e.g. for an aditionnal sirene), as I already stated.
Apparently, some manufacturers also have "switched input modules" that allow an external device to trigger the smoke detectors (this is apparently used to interface with sprinkler systems or other builing alarm systems).
For example:
https://www.eielectronics.com/product-selector/80-product-list/189-accessories-hard-wired.html
So with one module of each one can interface both ways between an arduino and a set of smoke detectors. This also addresses MarkDerbyshire's concerns btw since these are pre-made modules so you only need to wire them up.
The PCB of the "switched input module" simply looks like a transformerless power supply combined with a battery power supply. So probabbly not much going on there except putting 9 VDC on the interconnect wire.
Another option is of course to open the fire detector and look up the datasheet of the smoke detector IC being used.
I guess this more or less answers my initial question.
The 9 V DC interconnect seems to be more or less a de facto standard although several manufacturers deviate from it (e.g. Kiddle running a digital protocol over the interconnect wire, and I have also seen 24 V DC fire alarms with an interconnect which might run 24 V DC rather than 9 V DC). Most manufacturers of course recommend to only interconnect with their own devices.
However, aditionnal info is of course still welcome.
Thanks to everyone who contributed.