It should be noted that "Red Alert: Remastered" is open source (but not free as in beer, probably not free as in freedom either).
If you buy "Red Alert: Remastered" on Steam, there's a directory that lists the entire source code of Red Alert: Remastered.
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They probably did that because OpenRA was so popular, and that people would be curious how the original was also coded. The "Remastered" edition only came out in 2020.
Warning: Red Alert: Remastered includes a bunch of x86 assembly files for 64-bit MMX SIMD. Some ugly, 90s-era assembly language there for pathfinding...
They licensed it under the GPL3 so it's free software. You can also get it also from EA's GitHub:
https://github.com/electronicarts/CnC_Remastered_Collection
They only released the source to the game logic, not the full engine.
And of course, the game data is not free.