The whole "Which universe is this?" bit seldom ends with hard answers. A lot of that has to do with the fact that comic book universes are notoriously fluid and often go through "resets" that coincide with new writers, etc. This is pretty evident when you look up Earth-616 (comics)
The reality of Earth-616 has gone through eight different incarnations, all triggered by different instances of multiversal renewal, which constitutes in the destruction and re-creation of everything there is.
616 has always referred to the continuity that Marvel comic books are taking place in. Earth 616 shares our "real world" history to some extent (World War II, etc).
The numbers have allowed for the telling of other stories in other universes. When the MCU started out, people referred to it as Earth 199999 to avoid confusion (the MCU diverges greatly from the comics in some places). At least one official Marvel publication gave it that designation. The catch is that nobody in the MCU has ever referred to it as that in-universe. Their continuity is referred to as drumroll please... 616
Spider-Man: Far From Home
Nick Fury: Mr. Beck is from Earth. Just not yours.
Quentin Beck: There are multiple realities, Peter. This is Earth Dimension 616. I'm from Earth 833.
And Doctor Strange and the Multiverse of Madness
Christine 2: Our universe is 838. And we've designated yours 616.
What this means is that you now have the Comic 616 and the MCU 616. That's still confusing, so when someone says 199999, they unambiguously mean the MCU, which is still different from the comic continuity.
Put another way, there are multiverses of multiverses. Try not to let that concept drive you to madness...