NEM Glossary - Blockchain Layer 1
Blockchain Layer 1 can be called the operating system for the decentralized internet. And layer 2 is software aplications running on this operating system. Layer -1 is the term used to describe the basic architecture of the blockchain, both physical and software.
Physical Layer - can be described as the physical network infrastructure required to keep the blockchain running. In many ways, the blockchain itself can be seen as just another layer on top of the Internet, as without it the blockchain would not function. Right now, blockchain needs the Internet to work. Thus, the physical layer consists of protocols, connections, hardware, miners, and everything else that lays the foundation for blockchain implementation.
Programming layer - This is the layer responsible for certain protocols, consensus mechanisms and everything else that provides the functionality of the base layer of the blockchain and the associated cryptocurrency.
For example - Bitcoin is a Layer 1 network and the Lightning network is a layer 2.
You can find out which blockchain protocols belong to layer 1 at this link: https://blockchain-comparison.com/blockchain-protocols/