EA have restored and released the full source code for several antique Command & Conquer games under the GPL license, in a red letter day for series modders.
The games in question are the original Command & Conquer (aka Tiberian Dawn), Command & Conquer: Red Alert, Command & Conquer Renegade and Command & Conquer Generals, plus the latter's Zero Hour expansion.
That's a lot of refurbishable strategy game to get your head around. All this is reportedly the result of EA commissioning one particular modder, community figure Luke "CCHyper" Feenan, to restore the old C&C source codes, as described at fearful length on Reddit.
If you're not familiar with him, he was involved in bringing the C&C Ultimate Collection to Steam back in March 2024. RPS Tank Driver-For-Hire Steve Hogarty called that "the treatment these genre-defining games deserve".
You can find the source code for Tiberian Dawn on this Github page. The Red Alert code is over here, while Renegade is here, and Generals/Zero Hour is thisaway.