The Dell OptiPlex 7090 SFF supports 11th Gen exclusively via its Q570 chipset

The 7090 SFF uses Intel’s Q570 chipset on an LGA 1200 socket and only supports 11th Gen Core processors. You can’t fit 10th Gen Comet Lake or 12th Gen Alder Lake chips — the Q570 is wired specifically for Rocket Lake, which narrows your options if you find one used. Make sure the CPU you want is on Dell’s supported list before buying.