I am a special effects artist for Disney, and I have worked with them for many films (not Endgame). I was hired back in 2018 to work for the Spider-Man Homecoming sequel, later titled Far From Home. I helped the team to CG many scenes in the film, some non-important while some important. I won't get into any spoilers from the film itself, but what I have seen will blow your minds (I haven't seen the whole film just different parts). I won't spoil what I saw from the film, but I will tell you guys what the post-credit scene is. In the paragraph below are post credit scene spoilers so be warned. The reason I saw the mid-credits scene was because I had to fill in CG on a character.
There is a mid-credit scene and a post-credit scene, but only one is super important, which is the mid-credits. I haven't directly seen the post-credits scene, but another artist told me it was something comedic with Happy and Peter, which has an insignificant impact on the overall story arc.
The mid-credit scene starts off in a dark room, with chilling and villainous music. A defeated and sad looking Mysterio looks out the window from a skyscraper at night time. The view of the shot is within the building looking outwards, so you can't see the building itself. A voice is heard in the background (which sounds Russian), which says something like "He beat us once, but soon he is going to pay". Mysterio turns around and the voice is revealed to be from the Chameleon in his costume which is the character I helped to CG in this scene. Mysterio then says "Dimitri, thanks for having my back out there". Chameleon replies with "don't worry, there are more of us". Right then, the music gets darker while the Vulture (similar outfit as homecoming with a few modifications), Scorpion (Outfit similar to comics but few differences), Shocker (Different outfit than homecoming, it looks like the red and yellow suit from comics), and a man in a business suit walk in from another room. The businessman gathers the villains together and says "I have a proposal". The camera pans out to the outside of the building, which is the old Stark/Avengers tower which has the name "OSCORP" in large green font, at the same spot the Stark logo was, and shows it for a few seconds. The scene ends here. It was about 45 seconds but I have only seen the whole clip twice. I couldn't tell if the businessman was Norman Osborn, or if he was simply an assistant, and I didn't know the actor playing him (I don't know much about Actors, he was probably a B-lister). This scene obviously sets up the Sinister Six which will probably be further explored in the MCU. My task for this scene was to CG the Chameleon's face, mainly during his small talk with Mysterio, therefore, I wasn't shown the whole clip until it was all done.
And again, I haven't seen the whole movie, so I don't know the entirety of what happens, but I wanted to share this scene with you guys.