In summary, I got two different 1.2V memory models work at 2933 in XPS 8940: - Crucial CT32G4DFD832A.16FB1  Amazon marked it as single rank, but it is actually dual rank.  The part number is very very close to the Dell origin parts. Dell shipped with low end crucial memories, and sell them at more than double the market price. - Mushkin MSL4U320NF32GX2  Amazon has a bunch of them, different models, but not too much difference (with or without heat sink), any of them should work. Both works fine with the 2 * 32GB, and/or together with the original 16GB come with the XPS computer. Both are CL22. Both are pretty cheap.   The other popular gaming RAMS on the market at 1.35V + can worker too, but won't run at 2933, can only run at 2400.
PS 8940 Special Edition, Intel i7 10700K,  came with 16GB RAM 3200, runs at 2933. I am trying to upgrade to 64GB ram. tried 3 popular brands, none of them can run at 2933 or 3200. - Corsair Vengeance LPX 2x 32GB, DDR4 3200,  can only run at 2133 - Crucial Ballistix 3200 MHz DDR4 2x32 GB, can only run at 2400 - HyperX predator 3200 2x32GB, can only run at 2400.   All these third party RAMs requries to turn on XMP in the BIOS. But Dell's BIOS does not have such options. Dell's original parts are so expensive,1 x 32 GB cost 361.9$. Third party RAM 2 x 32GB cost ~ 280$. And third party rams come with the fancy RGBs and heat sinks. Anybody has luck to get third party ram work at 2933MHz?
XPS 8940 SE, third party RAM cannot run at 2933MHz