Seatbelt at the time of the collision?

As both airbags were deployed you will need to replace both of them, plus you will need a scan tool to go into the airbag control module and see which sensors triggered it to deploy the bags and replace those as well. Because it did deploy both driver and passenger bags, I'm going to say that both seat belts deployed as a result. Use a scan tool to check all of the sensors as well as the occupant classification system and the remaining air bag modules.
The newer SAS systems like yours have a few options for sensors and deployment based on things like seat sensors, door sensors, speed inputs and more, with the scan tool you can "see" the faulty parts and tell when they are repaired. However, because some items are optional depending on where it was built and which option package it has, there is no real way to tell which sensors triggered without the OE level scan tool.

Does the Mazda cx3 2018 and the Mazda 3 2018 have the same size engine subframe cradle?

When replacing the passenger front airbag, do i need an entire dashboard replacement or i can just put in the new airbag in the current dashboard ?

The subframe is different because of the 4WD system. For the airbag it depends on the damage. If it tore the pad, then yes you replace it. Usually, you end up replacing it because of the way it deploys.