I believe demons are fallen angels who joined Satan in his rebellion, and, as such, are much-more-powerful than any of us. However, they must obey God no matter how much they resist doing so, as did the ones Jesus ordered out of a man & allowed them to enter a herd of pigs. And I believe they can still possess people, if people either invite them, or do nothing to resist them, especially not using God's "armor" & "sword".
I believe Hitler allowed himself to be possessed for awhile by an especially strong demon by his dabbling in the occult in his youth. This demon manifested itself after Hitler's WW1 service, enabling him to suddenly become a demagogue & master manipulator, "messing up" only with the "Beer Hall Putsch" of 1923. From then on, he did about everything right, thru the conquest of France, but I believe God ordered the demon to end fully possessing him after that, as it wasn't yet time for the antichrist to come. But the demon still watched over him, & partially possessed him, preventing his assassination a number of times. More proof he was possessed-When something went wrong, such as the Nazis losing a battle, he was seen more-than-once, lying on the floor, kicking, screaming, foaming at the mouth, turning purple, pounding his fists on the floor, & gnawing the edges of the carpet, if one was present. Thus, behind his back, many of his generals called him"Teppichfresser" literally "Carpet-Eater".
When I was an unsaved teen, some friends invited me to attend a "Holiness" church service with them. it started out as an ordinary church service, but the preacher got louder & spoke faster, til he became unintelligible, & more & more of the congregation began raising their hands, then yelling & screaming, then dancing, & many lay on the floor, cutting dust angels. Though unsaved, I felt a great sense of EVIL in that place, believing I'd walked into a funny farm instead of a church. I quickly got out, got into my car, & left, without telling anyone goodbye.
Next day, my friends asked me why I'd vanished, & I told them. They tried to tell me it was Holy Spirit possession that'd occurred, & I said I'd sooner avoid THAT!
I actually think that experience kept me from coming to Jesus sooner, as I associated many churches with that ghastly spectacle. So, the demons had done their job for awhile!
But yes, I believe it still occurs, but any Christian can be an "exorcist" by invoking Jesus' name; no special person nor ceremony needed.