In Electroneum (A monero fork), there is a prefix set such that every wallet address begins with ‘etn’ . Where can this be set in the monero code and what is the purpose of it?
Transaction prefixes are set in the cryptonote_config.h file in the src subdirectory of the source code root directory.
The purpose is so coins using the cryptonote protocol can be differentiated from one another. Monero was a coin among a range of other created in 2014 along with Bytecoin and others. It's changed a lot since those days with the help of this community, but that prefix structure is still there.
To set it, you'd have to convert your desired prefix into the equivalent Base_58 value. Cryptonote actually created a tool on their site that allows you to do this: https://cryptonotestarter.org/inner.html