Why is Trump president?

Regardless of varying personal opinions about Donald Trump, he is the legitimate forty-fifth president of the United States. He won the 2016 presidential election by capturing 306 electoral votes (270 or more are needed to win), even though he lost the popular vote by almost three million votes.
It is important to understand how the United States voting system works to completely understand how one candidate can lose the popular vote but win the presidency. Donald Trump is not alone in losing the popular vote but winning the presidency. John Quincy Adams (1824), Rutherford Hayes (1876), Benjamin Harrison (1888), and George W. Bush (2000) all lost the popular vote but won the electoral vote and thus the presidency.
You may ask yourself, “How can someone lose the popular vote and win the presidency in a democracy?” The answer is that the United States is not a direct democracy—the United States is a democratic republic. As a democratic republic, United States citizens both directly and indirectly choose elected officials. When voting for city council members, state senators and house members, governors, and federal-level senators and house members, United States citizens directly vote, meaning the candidate that wins the popular vote wins the seat. This is not the case for the presidential and vice presidential election. The electoral college determines the winning candidates of the presidential and vice presidential election.
What is the electoral college? The founding fathers established the electoral college in the constitution. It is made up of 538 electors with varying numbers of electors representing each state and the District of Columbia. The electors represent the president and vice presidential candidates. When citizens vote for a presidential and vice presidential candidate, they are actually voting for that candidate’s electors in their state. Those electors then cast their vote for their candidate. All states but Nebraska and Maine are “winner-takes-all” states in that whoever wins the popular vote in that state wins all electoral votes for that state. Nebraska and Maine use proportional representation, which means that electors are divvied up by the percent of the popular vote each candidate receives in that state.
The original purpose of the electoral college is two-fold. First, the founding fathers believed that the electors would more aptly choose a qualified president and guard the citizens against exploitation. They also believed that the public was not necessarily able to make the right choice and that by having the electoral college meet only once in determining the winner of the election, they would be less likely to be corrupted by any one candidate. Second, the electoral college was one of the compromises made at the Constitutional Convention to create equal representation in the government for the small states.
After the 2016 election, many people are calling for the abolishment of the electoral college.  Those calling for its abolishment say that the electoral college creates a system where the person with the least amount of votes could still win the presidency, it is complicated and discourages individuals from voting, and it gives small states and swing states more influence in the election. Those who support the electoral college say that it helps protect the interests of states with smaller populations, supports the two-party system (which they say helps create stability in the political system), and gives more power to the states.
Donald Trump successfully won the majority of electoral college votes by relying on the states he knew he could win (traditionally red states) and appealing to the disenfranchised voters in blue and purple states, effectively winning the popular vote in those states (even by a slim margin) and taking those electoral votes. Political pundits say that his win was a surprise because those on the left were using traditional political tactics and past successes while the Trump campaign tapped into the disenfranchised voters in traditionally blue states—most especially, working, middle-class white men. Election polls that have accurately predicted the winner failed to do so in this election. The 2016 presidential election was a turning point in American politics that has sparked entrance of new and fresh blood in politics and review of what some believe are antiquated aspects of the electoral system (like the electoral college). The 2020 presidential election shall be interesting to witness.
https://www.archives.gov/electoral-college/about