The narrative the 2020 Democratic hopefuls are presenting, is effectively that the government can give everyone all this "free" stuff by taxing "the rich" and "mega corporations" their "fair share" in taxes. I put all of those items in quotes because nothing is free, how much you have to earn to be considered rich or a mega corp is never defined and neither is how much in taxes you have to pay to have paid your fair share.
The reality is the various plans presented by those politicians add around $40T in federal spending over the next 10 years and even under the rosiest of assumptions raise only $9T in revenue by taxing "the rich" and "mega corporations" their "fair share" as they desire. More realistic assumptions about those tax increases put the number closer to $4T in additional federal revenue.
No Democratic hopeful has presented anything even resembling an actual plan which raises the level of revenue necessary to fund all their wonderful plans. I firmly believe these politicians know their plans are not on feasible ground from a financial perspective and they're purposely preying on emotions (envy and fear, mostly) and misleading the public to try to gain votes. And when you question them on their math skills you're told to stop spewing "Republican talking points." Even if it were a talking point issued by the hypocritical Republicans, does it somehow make the question any less valid? No, the Democrats would simply prefer to avoid answering such a question.
If you want to read more about how those numbers were calculated and how they break down, I highly recommend reading the article posted by Brian Riedl over at the Manhattan Institute.