Here’s 538 page for Biden v. Trump polls. When they launch their forecast model – I think probably around June, but possible sooner – these will be the main input. High-quality state polls will have the highest weight (especially if they focus on likely voters), but national polls will be included.
(Just to remind everyone yet again: “75% chance of winning” does not mean “75% of the public favors this candidate.” It means, in 1 out of every 4 rolls of the dice (that’s a lot!), this candidate loses.)
(Ignore the “28 Jun 18” in the heading – that must be when they happened to set up this web page. The latest polls are from the past few days, and suggest the race is a tie – but this is very early days.)