It was the Romulans that created the Vulcans for numerous reasons (ranging from military uses to cheap labour to procreation to creating men and women that would act and behave logically in all aspects of life). And they were initially based off of the ridge-less Romulans, who were originally pacifists on Vulcan. However, the Vulcans started warring with one another, to the point that Romulans abandoned Vulcan.
Since then, there has been an interest in reunification, mainly because the Vulcans are a threat to the galaxy as they are designed to engage in violent and apocalyptic warfare, and there is a desire to get them under control before its too late. It also explains their dislike of humans, and why they’re still humiliated by their defeat at the Battle of Charon over 200 years ago later: they lost to the synths they created, and even think, or thought that humans were synths created by the Vulcans. And therefore, naturally become hostile to anyone that allies with humans and Vulcans (i.e. Klingons), as they secretly think of them as synth sympathizers and under their influence. The self-imposed isolation the Romulans tend to go through has really been away to figure out how best to deal with the Vulcans. And the best they have been able to figure out is by deceiving them into accepting reunification.
The interest in the Borg cube is that the Romulans are going to an extreme length in regards to reunification as a last-ditch effort before the Vulcans malfunction, cease embracing logic, and destroy the galaxy. They are essentially learning about assimilation, and how to assimilate the Vulcans into a Romulan created collective, and how to switch off said collective. The Borg as a species are technically dead, due to the events in “Endgame” on VOY; it just took years for the damage to reach all of the collective.
The connection to Maddox is that the way he created Dahj and Soji is how the Romulans created the Vulcans. Meaning the Romulans have always known how to create an android like Data all along for millennia. However, the experience went badly for them (Vulcan rebellion), hence the fear that Data’s daughters will do the same thing again. So they decided to stop Maddox.
The Romulans, despite their long history of deceit and treachery, are actually the heroes of the story.
Also, the Romulans are going to learn by accident that species 001 was the Iconians, and that the Borg and Iconians are actually related to each other.