It’s not so much that he will refuse to physically leave the premises (he is a coward), rather it’s the lies and hate he’ll spew. If Twitter were smart, they’d kick him off their site now, because come November he will flood it nonstop with lies and disinformation (he loves to whine as it gives him attention).
This is a man who will not - cannot - admit defeat. He never has and he never will. He will call the election a hoax, fraudulent, rigged, and inaccurate. He will claim that voter registries were inaccurate, and that they contained of dead people, duplicate or triplicate entries, people who moved, (etc.) which allowed “certain voters” to vote more than once. He will claim that certain “people” were out to get him so they rigged the election against him (he is paranoid). He will assert that he was right all along about mail-in voting being anti-republican (vote by mail WILL be a thing this November). He will blame and/or slander any and everyone he can think of (it’s his thing). He will call on his followers to “take back America”, resulting in rallies and violent protests. He will demand recounts, try to declare states results invalid, and just flat-out be a sore loser.