The President proposes the annual budget, and within the laws written by Congress and signed by the President he or she can propose increases, decreases, or new expenditures in the budget to implement the laws. As I mentioned in an earlier post, Trump is doing that with Medicare with what are effectively cuts, though he's also moving some responsibilities between agencies, so his changes aren't perfectly clear. The President can't, for example, implement Medicare for All by executive order. Mostly the President can make speeches and use influence within his party, or negotiate with the other party for support in Congress, but not much else. On the slim chance that Sanders is elected President, my guess is that he gets virtually nothing done, because he's actually an independent and not a Democrat; he's just running as a Democrat. Without the support of his or her's party in Congress, a President is a bit toothless. And, as Trump found out, even if your party has majorities in both Houses of Congress you may still be stymied if you try to influence Congress to move in ways they don't really want to go. For example, his wall wasn't funded. Or they might do some stupid stuff you don't like but you're not in the mood to fight about, for example the inane 2017 Tax Cut and Jobs Act, which Trump ended up signing anyway.
Congress for the time being seems to be doing nothing about healthcare. And it's easy to understand why if you just spend a couple of hours watching CSPAN. Even if Sanders gets elected I'm convinced Medicare For All is dead in the water. Or it'll have so many special loopholes and exclusions that it would be unrecognizable by time it's ready to be signed. And, given the purist that Sanders has been for decades, my guess is that he would veto such messy legislation.
Just my opinion, but I think that the pharmaceutical companies have pissed off both parties so much, even Trump has dissed them publicly and threatened action, that once the election is over something could get done to control prices. If I were a betting man, I would thinking the odds are 2:1 in favor something punitive by the end of 2021. If Trump is re-elected I think the chance of much beyond that seems unlikely. Maybe some legislation to prevent so-called surprise medical bills.