The troublemakers on the world stage banging the war drums have something in common - most are corrupt leaders using war as a diversionary tactic and means to cling to power. Another thing they share is viewing war as an alternative method of growing dwindling populations. Instead of the Russians and Chinese encouraging citizens to have more kids, they are trying to boost their populations nefariously, attempting to forcibly annex neighboring countries like Ukraine and Taiwan.
As for what a world war today would look like, and also why we should be doing everything possible to avoid such a thing, it would look a lot different from previous large scale conflicts. If, for instance, you think it would be waged "over there" and that civilians could kick back, watch it safely on TV, think again. According to the FBI, China and Russia would hack into American infrastructure, cut off our electricity and water supply among other things, causing widespread chaos, looting and starvation. I don't think most people would survive more than a month, and that would be without anyone using nuclear weapons. War between nuclear powers is an insane thing to pursue in this day and age, and anyone in their right mind should be working for peace.