No, they develop at least one with minimum, basic features their formal spec depends on. Then another. Then another. Then another. Then, they have four.

Alternatively, several of them develop several components in parallel with constant communication to keep them in sync and with a spec. These have the minimum, basic requirements.

Then, there's security-focused products that... going by your statement... start with a formal spec that ignores basic, integrity checks then develop four implementations simultaneously also missing basics. "Not a great plan." (Tony Stark)