Coming up with multiple vaccine candidates was fast (it took days to weeks in this case). That's partly because some approaches are straightforward (like growing virus and then inactivating it), partly because there are ready-to-go platforms for vaccine development (e.g., adenovirus-vector and mRNA platforms), and partly because there's been work on closely-related coronaviruses (SARS-CoV-1 and MERS, from which people already knew how to genetically modify the spike protein to remain in its pre-fusion state).

In short, there were several vaccine candidates within days to weeks of the genome being decoded.

What takes time is testing the vaccine candidates for safety and effectiveness. Companies normally go step-by-step. They run a phase-I trial, then evaluate the results and decide whether to go on to a phase-II trial. If they run a phase-II trial, they again wait until the results are in and have been evaluated before moving on to a phase-III trial. That reduces financial risk. In this case, companies began preparing phase-III trials before the phase-II trials were even completed. You can begin enrolling people into the trials and producing the necessary doses before you even know whether the phase-II results are any good. One of the reasons they could do that was because the government was taking on the financial risk.

Technically, Moderna's phase-I trial is not even complete yet: [1]. It runs until November 2021. But Moderna moved forward onto the next phase as soon as it had enough data from the phase-I trial to justify doing so (I assume this meant some combination of safety and efficacy data).

1. https://clinicaltrials.gov/ct2/show/NCT04283461