A disposable vape is three components in one sealed body: a battery, a heating coil, and a reservoir holding the cannabis oil. Everything else about the format follows from those three and from the decision to seal them together.
The coil is the part that matters most
The coil heats the oil to the point where it turns to vapor. How hot it runs, and how evenly, decides most of what the draw actually tastes like. Terpenes are volatile and heat-sensitive, which is the same reason they are lost during drying and curing, so a coil that runs hotter than it needs to will cook off exactly the compounds carrying the flavor. This is why hardware quality is not a separate topic from oil quality; it is the same topic at a different moment.
Why a disposable is sealed
In a cartridge-and-battery setup the two halves are bought separately and screwed together, so the same battery serves many carts and its contacts, voltage and age vary. Sealing them means the battery is matched to the oil and the coil it will actually run, and the airway is never exposed between uses. Leaf & Vine states its own version of this directly: each pen contains a battery designed to last the life of the pen.
The trade is the obvious one. When the oil is finished the hardware is finished with it, so a disposable is a device with a scheduled end rather than a platform.
Inhale activation
Most disposables have no fire button. A pressure sensor detects the draw and powers the coil for as long as it lasts. There is nothing to hold down and nothing to leave switched on, which is the whole point of the format.
What actually goes wrong
Cannabis oil is viscous and gets more so when cold, so a pen left somewhere cold draws hard until it warms back to room temperature. Heat is the opposite problem: a hot car thins the oil and encourages it to move where it should not. Neither is a defect in the pen, and both are avoided the same way, by keeping it at ordinary indoor temperature.