I have a question regarding the Order of the Phoenix film, in the end the team is next to what seems like a portal, and the order of the phoenix even enters from there. What was it exactly? Once Sirius Black dies, his body starts to float into it for some reason, and we never see his body again, it almost looks as if the Avada Kedavra spell had made him disappear..
The Order of the Phoenix dont enter through the Arch. They apparated in through the door that Harry and the group fell through at the top of the room. Then they flew all around the room.
In the books the Order just come running into the room through doors and even Dumbledore walks in like a BAMF and immediately makes Lucius and the other Death Eaters his *****es. In the books only Voldenort knows how to fly around like smoke. Yates tho gave the ability to any and every adult wizard.
As for the Arch the books never explain it very much. The ministry is built over it. And it seems to be at the very heart of the Department of Mysteries which is where Unspeakables study things like time, death, and other powerful ancient magic. The Department of Mysteries is a bit like our Area 51. Highly classified and top secret. That is why those who work there are called Unspeakables. They arent allowed to ever tell what they do in there.
The Arch is very ancient whatever it is. In the books when Harry and the others first arrive they wonder through a number of rooms. One has giant planets in it, another has a giant tank with floating brains in it that have thoughts coming off them like tentacles that can grab onto you and hurt you, another room is full of time turners. There is also a spinning circular atrium with doors all around the room and one of the doors is locked. Harry tries to use a knife sirius gave him that can pick any lock. The door's lock melts Harry's knife. Later Dumbledore tells Harry that in the Department of Mysteries is a room filled with the most powerful magic of all. He says that the power is love. Apparently love is an actual magic that is studied in the Harry Potter universe, and it is what gave Lilly's sacrifice the power to protect Harry from Volemort when he tried to kill him when he was a baby and what allowed Harry to protect all of Hogwarts from Voldemort when he sacrificed himself to Voldemort in the Forbidden Forest.
Anyways, back to the Department of Mysteries, they eventually stumble into the room with the Arch. Harry and Luna can hear voices coming from behind the Veil and it seems to have some sort of pull on Harry and Luna. Harry most of all. He even forgets why he is at the Ministry and approaches the veil without realizing it. Hermione cant here the voices but she gets really freaked out begging them to leave the room and she is able to get Harry's attention off the Arch and he remembers why they are at the Ministry and they leave the room and end up in the Prophecy room.
After Sirius falls through the Veil Lupin is trying to restrain Harry and Harry is screaming that sirius is alive. Lupin tells him that he is gone and that where Sirius has gone he cant come back.
So the likeliest thing is that the Veil is literally a doorway to the otherside. There has been all sorts of speculation about it in the fandom. Some have speculated and I also think this is a possibility that this portal is the origin of magic. There may even be other Veils in other countries that foreign Ministry's of Magic are constructed on top of. Unfortunately, or not depending on how you feel about it, Rowling never explained the Arch. So your guess is about as good as mine or anyone elses.
My advice, if you dont have time to read all the books or even all of OOTP, take the time to read the climax of OOTP. Start at Chapter 34 "The Department of Mysteries" What happens at the Ministry in the books is...well its a lot more. And there is a lot to the phrophecy that is left out of the films. An entire chapter is given to Harry and Dumbledore's discussion about the prophecy in his office after they return to Hogwarts and a lot is covered and lets just say a number of Dumbledore's prized trinkets and possessions are thrown and tables flipped. Its interesting.