https://docs.cypress.io/guides/guides/command-line#Exit-code documents that cypress basically has three exit behaviours.

Cypress succeeds means exit code 0

Cypress fails (complete run failure) exit code 1

Cypress: Specs are failing means exit code is amount of specs failed

But what if just one spec failed.

You could not differentiate if the whole run just failed or just one spec based on the exit codes.