The Ethereum project is one of the most developed and accessible blockchains in the ecosystem. Ethereum was first described in 2013 in a white paper written by Vitalik Buterin. Ethereum was developed in four stages: Frontier, Homestead, Metropolis, and Serenity.
The code is written in the Ethereum blockchain. When the code must be changed, it can be done with a soft fork or with a hard fork. With a soft fork, there is a backward compatible, not with a hard fork. For example, Constantinople was a hard fork. It introduced several EIPs (Ethereum Improvement Proposals).