Hello
Doesn't look like its been discussed previously, but a large chunk of the source for the first two Command & Conquer games has been released at https://github.com/electronicarts/CnC_R ... Collection to support modding the new remastered versions. The remastered versions themselves are not entirely open source engines, the front end for the dll files build from the open source code is a closed source product.
However the code drop contain most of the code from the original builds of the game and myself and a couple of other people are trying to fix the code up to run the original game content at https://github.com/Vanilla-Conquer/Vanilla-Conquer. Currently we have working builds for windows and are working on making the code more portable to create portable open source versions of the game engines to run the original game content that was released as freeware years ago.