There was a very clear reason for the disconnect between Trump's treatment of Biden, and his treatment of Warren and Sanders, despite what you want to peddle here. Biden went on video and bragged about withholding billions of dollars from Ukraine unless they fired the prosecutor that was investigating the energy company on which Biden's son sat as board member (and legal counsel?).
Now, you can make the argument that that was all on the up and up. You can even believe that. It may even be true!
But it's also a conflict of interest, a HORRIBLE look, and probably worthy of investigation.
Neither Warren nor Sanders were on video bragging about that apparent conflict of interest. Thus, there is your disconnect between how Trump treated Biden verses the other two.
Biden is out there blathering on about loving it when kids sit in his lap and stroke the blond hair on his legs. Don't flatter him when it comes to his danger against Trump.