You can see the dates when the datasets they incorporate into their model are available by hitting the Release Dates tab below the chart.

https://www.atlantafed.org/cqer/research/gdpnow?panel=4#tabfa9ccf1bede04c3b8a3ac751883e58a90
(Hopefully that link takes you right to it.)

They probably use some historical data from the previous quarter and prior year to guesstimate values in their model prior to the actual datasets being released. Theoretically it should get closer to what the prediction should be as more of the data is added to their calculations (i.e. as the actual release date by the BEA nears).

I happened to run across the GDPNow website in some notes I was going through, took a quick look at it, and popped it up here without reading much of it, so I'm glad you did.

Pre-Covid, it was interesting to compare the Nowcast and GDPNow numbers - they were often quite a bit different even though they are both data driven models predicting the same metric.

For example (from the final two updated Nowcast quarters):

Quarter   Nowcast   GDPNow   BEA(adv)   BEA(last)
2021Q1       6.66         7.87           6.4             6.3
2021Q2       3.12         6.43           6.5             6.7
Average       4.89         7.15           6.45           6.5


Which just goes to show that this is not an exact science - even the BEA's final, revised numbers are estimates.