Director Hayward talks about how hard it was to keep the lights on during the 5 year snap. So we know that S.W.O.R.D. was obviously active during this time as well as other military agencies. These agencies have satellites and drone surveillance, inc Starks own satellites.
The Avengers HQ is far from the big cities but there must be some people living nearby. When Thanos's ship appeared in the air and blew the HQ to hell somebody must have noticed or at least heard it. When reports came in you can bet S.W.O.R.D. was monitoring the situation to find out what was going on, and prob got some debrief of the situation from the Avengers themselves.
Considering Monica, Darcy and Woo are discussing this 3 weeks later is not farfetched that they knew some details from the battle. Even the general public knew what Tony Stark did to save them with banners and murals in his honour all around the world in far from home 8 months later.
Steve Rogers himself mentions Thanos in the therapy meetings. So the Avengers told the world what happened after Thanos snapped his fingers. Those who survived in the 5 years knew what happened to the dusted.