Well, since it's technically a different topic from last time and it involves math, I wanted to address this one particular thing.
To be fair, you could say that about any primary, really, both Democratic and Republican. Turnout always sucks in primaries relative to the general election. Even in the general election at least 40% or more of American adults don't vote, though that's mainly because roughly 30-33% of Americans over 18 weren't registered to vote in any given election year in recent history. Turnout of registered voters in presidential elections is high, but it's lower for midterms and especially primaries.
For example, only 5.8% of Americans (7.6%, if we only count the over-18 population since they're the only ones that can vote) voted for Obama in the 2008 Democratic primaries, and that was the best year in a very long time for turnout in the Democratic primaries. Obama and Hillary had about 35 million votes combined in the 2008 primary. In the general election, Obama received 69.5M votes, nearly twice as many as actually voted in the Dem primary. In total, about 59M people voted in both the Democratic and Republican primaries combined, yet Obama and McCain had a combined popular vote of about 129.4M, over twice as many as actually voted in the primaries. That year, only about 36% of registered voters actually voted in the primaries, while about 80% of them voted in the general. Even in New Hampshire, which has an open primary and typically the highest turnout among all the primaries, the number of people that showed up to vote in the primaries was about 26% lower than the number of people that showed up to vote in the general election. With such low turnout, only a relatively small number of total adults or even just total registered voters will actually vote for the eventual Democratic or Republican nominee in the primaries. So no nominee ever reflects "the will of the people."
In fact, you could argue that no elected representative at all actually reflects "the will of the people." Only about 30% of the adult population voted for Obama in 2008, and he had the highest popular vote count ever in absolute terms (Nixon and Reagan have him beat in terms of share among post-WW2 candidates, but not by a significant amount). Well under half of the voting-age population in nearly every jurisdiction is responsible for putting any representative in office, be they a governor, a Senator, a House member, a mayor, etc., because of the aforementioned relatively low turnout in the U.S.
Perhaps, "will of the voters" is more accurate. After all, only those that can and do vote actually have a say in who wins elections.
Granted, even that's often not accurate due to voter suppression (something that seems to always happen in GOP-led states) and some contests having significant distorting effects either because the electoral rules are inherently unrepresentative themselves or because someone is bending the rules to their advantage. Namely, House districts are subject to gerrymandering (which is why we need anti-gerrymandering laws or, preferably, proportional representation), and Presidential elections have the Electoral College (which needs to be abolished in favor of a national popular vote). But it is mostly true for Senate elections and gubernatorial elections. Whoever earns the most votes in those contests wins. There's no way to gerrymander states because their borders are permanently fixed, and there are no state-level equivalents of an electoral college for selecting governors, so assuming no voter suppression or other shenanigans the winners of those elections are those who legitimately earned a plurality or majority of the votes. There is still room for improvement even in those contests, and I think every election needs some sort of ranked choice voting system, but in a contest with only two candidates and no way for parties to choose their voters, the candidate with the most votes wins.
As for primaries, they're... complicated. It's hard to say if things like their staggered nature or the existence of caucuses are distorting effects (and then there's the debate on open vs. closed primaries). And FWIW, neither party is legally obligated to even hold primary elections in the first place, as they are private organizations that can set their own rules regarding things like membership or candidate selection. That voters have any input at all on the matter of who represents the party on the ballot is a courtesy afforded to us, and almost certainly due to increased pressure for more democracy. Primaries being the, uh, primary means of selecting nominees is something that's very recent, and it was within my lifetime that both parties had primaries/caucuses in all 50 states.
But with that being said, looking at the actual delegate allocation rules set by each party, I think the Democratic primaries have the better rules as they award pledged delegates on a roughly proportional basis, and so there are arguably no significant distorting effects in the actual electoral rules. The pledged delegate count isn't always exactly proportional to the popular vote count (there is that 15% threshold rule), but the odds of the pledged delegate count being way off the mark compared to the popular vote shares are very low. Kerry won 61% of the popular vote and 62% of the pledged delegates in the 2004 primaries. In 2008, there were some oddities (Obama taking his name off the Michigan ballot, and four caucus states never actually releasing popular vote counts), but by most accounts Obama barely edged out Hillary in 2008 in the popular vote and also just barely beat her in the pledged delegate count. Hillary had 55% of the pledged delegates and 54% of the popular vote in the 2016 primaries. And while the current primaries aren't over yet, the popular vote and pledged delegate ratios between Biden and Sanders are nearly dead-on proportional (though both are currently over-represented compared to the other candidates, though that's almost certain to change as results continue to come in from more states). So far, in at least the past 20 years the outcome of Democratic primaries has closely reflected the will of Democratic primary voters, and it's highly unlikely that the national popular vote winner will fail to earn the most pledged delegates and thus the nomination. Is there still room for improvement? Most definitely (and I wouldn't mind debating possible reforms to the primary process at a later time), but the Democratic primary rules have done a good job of making sure who wins is actually the biggest vote earner.
Meanwhile, the GOP primaries have a mish-mash of rules, including many states that are winner-take-all, so there is a significant distorting effect similar to that of the Electoral College. For example, Trump won 58.3% of the delegates in 2016 with only 45% of the vote, which is quite unrepresentative. Double-digit disparities between popular vote and delegate counts are the norm because of all the WTA states. However, ever since primaries have been the main means of selecting party nominees, the candidate with the most popular votes became the GOP nominee, and over the past 40 years all but two winners won with an absolute majority of the popular vote (McCain and Trump only had a plurality). It is conceivable, however, that in a close enough race the runner-up could earn the most delegates and thus win the primary simply by winning enough of the right states. The odds of such an outcome are far more likely than with the Democratic primaries.
The way that the Democratic primaries are conducted are better than the Republican party to be sure. Primaries need to start being treated as importantly as the general. We need to revamp how and when the primaries are held like having all states vote on the same day and having election day a national holiday, both primary and general election. I'm just suggesting things off the top of my head but I'm sure there's other ways that are better.