I’m not from the US, either. But from what i understand, the issues comes from the US having a “first past the post” voting system on a state level. The President is not elected by the percentage of votes, but each of the 50 states gets assigned a number of electors, based on their population. When a party/candidate has won the majority of votes within a state, they will receive all the electoral votes of that state.

Here’s a simplified example of how that works: Let’s assume 3 states with an equal ammount of inhabitants (let pop=1 million) and an equal ammount of electoral votes (let el=10)

State 1 has:
600.000 votes for candidate A (60%)
400.000 votes for candidate B (40%)

State 2 has:
200.000 votes for candidate A (20%)
800.000 votes for candidate B (80%)

State 3 has:
510.000 votes for candidate A (51%)
490.000 votes for candidate B (49%)

candidate A has received a total of 1.310.000 votes (~44%)
candidate B has received a total of 1.690.000 votes (~56%)
candidate B has won the popular vote, because most people voted for them.

However, candidate A won the majority in States 1 and 3. So candidate A will receive all 20 electoral votes of those states (which they won by only a comparitively small margin), whereas candidate B will receive only 10 from State 2 (which they won by a landslide).
As a result, candidate A will become the next president.