One of the staples of sci-fi and fantasy literature– and some historical fiction– is the sprawling, world-altering conflict with which the protagonist or protagonists are embroiled. With the possible exception of a Chosen One, your protagonist will be only a small part of an epic struggle unfolding around them.
Capturing scale without resorting to a head-on infodump takes a bit of finesse. A giant conflict like a world war or a battle against a supernatural Evil Overlord will probably take place on a large temporal scale as well as a large spatial scale. Furthermore, it’s likely that even if we’re just viewing the most recent showdown in a conflict– as we are in the original Star Wars movie, for example– that there have been a huge number of previous incidents, large and small, leading up to that moment.
One way to establish scale is to have these preceding conflicts built into your character’s world. Think of how we incorporate events and their aftereffects into our language– ‘9/11’, ‘Brexit’, ‘Watergate’– locations or dates or important names become a shorthand for complicated events that have indelible effects on the world around us. If you do that, the audience will absorb this information and its significance without requiring you to infodump directly. There are also plenty of opportunities for characters to argue over competing versions of the facts, or their own interpretations of what the established facts mean, or what is best to do in the situation. That’s a lot of opportunities to work in details and exposition about the larger world during plot-focused or character-focused scenes.
Another aspect of conflict at scale is the persistence of normalcy. There are actually two sides to this: first, normal routines, rituals, and daily tasks continue to persist, even after the larger conflict has started. Second, the state of conflict becomes the new normal, as people become desensitized to the threat, or even feel energetic and invincible upon surviving a calamity at a distance. People will adjust to new threats and adapt their routines around them with surprising adaptability.
Remember, your characters are in this for the long haul, so show them adapting to the ongoing conflict– the war, the invasion, the altered climate, or whatever the ongoing disaster is– and manuvering through the logistics of their daily lives as well as tackling the large-scale problems.
That said, it’s unrealistic that living in such an environment can be truely normal, especially for those in your setting who aren’t buffered from the worst by geography, wealth, or some other mechanism. Individuals have a wide range of responses to prolonged fear and chaos, based on their own experiences and if they are directly affected by the unfolding disaster. The targets of racial animus or the people living in the path of a natural disaster, for example, will have a much more traumatic experience than those observing or helping from further away. The group is likely to have a collective traumatic response as well as the individuals involved.
Writing a large scale conflict takes a lot of planning of the ‘big picture’ to keep everything internally consistent. I’d recommend keeping a notebook (real or virtual) with your worldbuilding notes, but there’s lots of methods– find one that works for you. Don’t be intimidated by the amount of worldbuilding work necessitated by such stories. Dedicated worldbuilding has given us some of the most vivid and enduring fantasy series around, and it’s a great tradition to be part of.