Do you have some kind of edge hardware next to each field device also? Or are those field devices natively MQTT capable?
There is no magic. An MQTT setup looks something like this:
[Ignition] <- MQTT consume - [MQTT Broker] <- MQTT publish - [Edge Device] - poll -> [Field Device]
With some variation for whether you actually need an Edge device or not, and whether you use the built-in broker from the MQTT modules or a separate one.
If the field devices don’t have built-in MQTT functionality then something has to be next to it polling the data and publishing to the MQTT broker.