Picking the states isn't super hard. At least 40 of the 50 states are virtually guaranteed to one party or the other in the current era of politics. That leaves roughly 10 states or less that are true swing states. But polling for these states generally gives you a good idea of how they are leaning. I bet you weren't nearly that accurate in 2016.
It's much more impressive to predict the percentages, which is what this chart reflects. Biden had large poll leads in MI, PA, and WI heading into the election in 2020. Yet Biden narrowly won all of those states. WI was decided by 20k votes.