In Avengers: Infinity War movie, we saw
Red Skull guarding the Soul Stone on the planet Varmir.
We also saw that he was flying/floating. Provided that he was a human, how could he do that?
In short, he's no longer human. The Soul Stone has trapped him and is basically making him into its guardian.
What was the decision making process to bring Red Skull back?
Joe Russo: Somebody had to be the gatekeeper to the Soul Stone and somebody had to distribute the rules to Thanos and Gamora. ... Who felt credible to the audience. And a character who has been trapped – the Red Skull has been turned into this wraith who now guards the Soul Stone and distributes the rules to whoever shows up there trying to obtain it. It felt like there was more credibility than just a random character. ... Ross Marquand did an incredible job of capturing not only the essence of his voice, but the essence of the performance – a haunted figure who is now this imprisoned spirit.