If you are looking for the source of heat, which comes from the chemical capsaicin in chili peppers, the statement that you will most often find goes something like this: The source of heat in a chili pepper is not the outer flesh of the chili, but rather the inner membranous ribs and the seeds. To remove some of the heat, remove all the seeds and the ribs. This is both incorrectly stated and the source of an urban myth about chili peppers. That is, that a lot of the heat, or capsaicin, in a chili pepper is contained in the seeds.
See also: How Do We Get Used to the Heat In Chili Peppers?
The fact is that the seeds themselves have no heat. However, the ribs do contain a good deal of the capsaicin heat of the chili. The ribs do not contain all of the heat, though, as is often claimed. The flesh of a chili does indeed have plenty of heat, and this is very easy to verify for yourself.
How much heat you remove by removing the ribs, a part which is called the placenta by botanists, will vary with the potency of the chili. These membranes are attached to rows of seeds in the cavities. So, the seeds will tend to have a lot of this dissepiment stuck to them and that is where the heat comes from that has been attributed to the seeds.
https://culinarylore.com/how-to-guides:roasting-chile-peppers/
The seeds are only hot by association with this material. If they were completely cleaned off, they’d have no heat. I have read, on occasion, that the seeds may “absorb” some of the pungency from their proximity to the ribs, but I do not know if this is really true.