Do cash debits and checks lead to automatic sales of securities or does it roll into an automatic margin loan?

If so, how do you decide what securities to liquidate?

If not, what’s the margin rates and is there a spread atop Fidelity’s rack rates?

I’m in the skeptical camp as well as none of this seems that’s useful vs the potential fee structure and risk profile. Anybody that wants this now can setup a checking account alongside their brokerage and manually sweep cash as needed. That also has the advantage of being in explicit control of what monies get moved.