The egg consists mainly of proteins, as well as fats and carbohydrates. If you speak the scientific language, there is a proucess of proteins denaturation - transition of a protein from the tertiary or quaternary structures to the primary. Schematically this process can be depicted as follows:
(AA)n = nAA
Where AA is amino acids.
Нowever, this is very schematic. There are billions of different chemical reactions occur during egg preparation, each of which cannot be described. It is also known, in particular, that during denaturation, disulfide bonds are destroyed and a product of iron sulfide is formed.