Collisions occur when two or more devices try to transmit signals on the same physical medium at the same time. This causes interference and data loss. Collisions can only happen at the physical layer of the OSI model, which is responsible for transmitting and receiving raw bits over a physical medium such as a cable or a wireless channel. The physical layer does not have any mechanism to prevent or resolve collisions. Therefore, higher layers of the OSI model, such as the data link layer, need to implement protocols to detect and recover from collisions, such as CSMA/CD for Ethernet networks.
References: Collision in computer networkingData Link Layer | Layer 2 | The OSI-Model