So MQTT is the light weight messaging protocol based on publish/subscribe model.
Originally OPC-UA was based on a request/response model, but I have read that it now also supports pub/sub extension.
I also understand that OPC-UA is a not really a protocol but a system architecture. Other than this fact, from the view point of protocol features (considering pub/sub only) what is the difference between MQTT and OPC-UA?
OPC UA PubSub extends the classic client/server communication pattern with a publish/subscribe pattern. OPC UA PubSub itself is not defined in terms of a specific protocol, instead a number of mappings to transports (and encondings) are specified.
An MQTT transport with a JSON encoding of the data is one of the mappings. This mapping defines a standard how to encode UA data models as JSON. It also defines a standard how to publish (JSON) meta data describing this data.
This allows to publish a UA DA datatype like an AnalogItem via MQTT and get describing properties like a Range or a Unit.
OPC UA also describes interfaces which allows to configure the publisher.
Details can be found here: https://reference.opcfoundation.org/v104/Core/docs/Part14/
To get an overview it is enough to read section 2, 3, 4 and the earlier parts of 5.
