Let me preface this by saying that I hope Joe Biden doesn't die for any reason.
Currently, 538's 2020 Democratic Primary forecast has >99% odds that Biden wins more than half of pledge delegates.
https://projects.fivethirtyeight.com/2020-primary-forecast/
What would happen in the 2020 Democratic Primary if Biden were to die during the primary?
There is an article on the question here, which mentions that it has never happened before, but it doesn't really answer the question beyond saying that the party finds a replacement.
I'm assuming the Democratic Party would nominate the Biden's chosen vice president had he won the nomination, but I'm more curious as to what would happen if he died before he won the nomination and hadn't yet picked a vice president. Would the second place finisher (Sanders in this case) become the nominee, or would other candidates that have dropped out have the chance to get back in? Would they vote on at who his campaign was looking at as a vice president (seems unlikely)?