I have a question? So, in DnD, what's the history on dragons? And how is it that all the dragons aren't extinct or killed all the other smaller races? Where do they live and how is it that they aren't literally everywhere?
What is the history of Dragons in D&D?
This really depends on your setting, in The Dragonlance and Forgotten Realms settings the dragons exist as presented in the Monster Manual, split into chromatic and metallic types and further divided by color and metal (no sign of the Gem dragons yet) with draconic gods being worshipped by humanoids. The Greyhawk setting has fog, mist, shadow and “Greyhawk” (steel) dragons and was home to Ashardalon. Dark Sun doesn’t have traditional dragons, defiler/psionicists of the setting may through a magical process transform into a “dragon” becoming ageless and powerful. Eberron has three dragon gods, Siberys, Khyber and Eberron and dragons in the Eberron setting are not so tied to the usual alignment of their types.
How is it they still exist?
Dragons are some of the most powerful creatures in most settings, at their most ancient they are gargantuan flying lizards with devastating breath weapons, they can strike debilitating fear into any who look upon them, many know magic and have a hoard of wealth and magical items at their disposal. Some cultivate agents and worshippers, are able to hide amongst humanoids and are incredibly strong and tough. Some are born smarter than most humans and grow craftier with age.
In my campaign, players will be dealing with a cunning green wyrm whose lair is within the boundaries of the neighboring kingdom but preys almost exclusively across the border in the PC’s home kingdom. The dragon knows that her victims cannot strike back at her without crossing the border and risking war. The neighboring kingdom isn’t bothered by the green dragon and will try to profit from this leverage should the PCs negotiate permission to enter the realm and slay it.
Dragons tend to find lairs in their preferred terrain, white dragons love snowy peaks and glaciers, blues prefer stormy coastlines, green dragons love being surrounded by vegetation, black dragons dwell in swamps and bogs while reds lair in warm mountain regions. Each chooses their lair with an eye towards security for their hoard and multiple escape routes. The area around a dragons lair is warped by its presence, the lairs of chromatic dragons become inhospitable and difficult to navigate, with various other effects depending on the color of its owner while areas around a metallics lair tend not to gain lethal effects but strange and wondrous changes.
How aren’t they everywhere?
Usually the enemies of dragons are giants and other dragons, aside from the enmity between metallic and chromatic dragons, chromatic dragons hate each other and see even members of their own sub types as rivals. Metallic dragons can usually be motivated to challenge or help adventurers against chromatic dragons that are becoming a menace, though use this carefully if you choose to, the pc’s shouldn’t be bystanders to such an awesome fight, they should be part of them. Any dragon who lives to become an ancient will have had to defend its hoard and lair many times.
The 4e Draconomicon on Chromatic Dragons
The 4e Draconomicon on Metallic Dragons
The 2e Council of Wyrms Campaign Setting
The Wyrms of the North article archive - the list scrolls down but that isn’t obvious if you are on mobile.