In Ms. Marvel episode 3, we learn that Dr. Erik Selvig has published papers based on his research on inter-dimensional travel.
Kamala (to Bruno): You're good at math. Do you know anything about interdimensional travel off the top of your head? Specifically that of the Noor dimension? That's N-O-O...
Bruno: Actually, that reminds me of a paper I read. I think it's by Dr. Erik Selvig.
Bruno studying Selvig's papers:
Bruno: So I plugged in the energy signatures from our tests on you and I ran a simulation.
Bruno: Seriously, remember the paper I was talking about? Theoretically, it's possible. [...]
It's likely that Beck or someone from his team of former Stark Industries employees (likely the writer, Guterman) has also read Selvig's papers, and crafted Beck's story about the multiverse and the "616" designation based on that. Relevant dialogue from Spider-Man: Far From Home:
Beck: To Guterman.
The story you created of a soldier from another Earth named Quentin fighting space monsters in Europe is totally ridiculous and apparently exactly the kind of thing people will believe right now. I mean, everybody bought it.
See this screencap of Dr. Erik Selvig from Thor: The Dark World (2013). Notice the information he's written on the blackboard in the background, and that "616 universe" is written on there. This was pre-Snap, years before the events of Spider-Man: Far From Home (2019).