I’m sorry you’re a little slow. I didn’t think it needed to be spelled out that he didn’t step down from the presidency (not that anyone can tell since he’s basically been mostly out of sight for weeks)
And yes, I understand how the 25th amendment works, and how the threat of it to remove somebody who refuses to leave the race can work, and did in this case. Biden did not step down of his volition. Everyone, but you apparently, knows this.