Chili peppers are typically crossbred to make them hotter by selectively breeding plants that produce peppers with higher levels of capsaicin, the compound responsible for the heat in chili peppers. Capsaicin is produced in the placenta, the white membrane that holds the seeds of the pepper, and the highest concentration of capsaicin is typically found in the ribs and seeds of a pepper.
Crossbreeding is the process of breeding two different cultivars of the same species together, in the case of chili peppers, this is typically done by hand-pollinating the flowers of one plant with the pollen of another plant. The process can be done by crossbreeding two cultivars of the same pepper variety or by crossbreeding different pepper varieties.
Once the seeds are produced they are planted and grown, with the plants that produce the hottest peppers selected for breeding the next generation. This process is repeated over several generations to produce peppers that are consistently hotter.
Additionally, mutations can happen naturally within the plant genome and this can result in a pepper that is hotter than the previous generation. These varieties are also selected for breeding.
It is important to note that creating a new variety of pepper through crossbreeding and selecting for heat can take many years and can be a difficult process. The heat level of a pepper can also be affected by environmental factors such as temperature, humidity, and soil quality, so it can be difficult to predict the heat level of a pepper until it is fully grown and harvested.