The merge, people say it conserves energy by 99.5 %

Can someone explain technically explain what is being implemented in the merge ? Was it an upgrade to the main.net ? Is it an upgrade to ethereum network ? Is it just a new way of validating each transaction ? Is it a new contract in the evm? What was changed in the code and for which project ? Can we see the repo of the project that will be released ?

PS I don’t know what I am talking about. Trying to understand what is going on instead of “ it conserves energy blah blah “