How is it determined who has won the presidency of the united states?

Through electoral college.

If you think your vote counts: think again. In electoral college, your vote could mean less if you live in certain states.

In electoral college, states are given a certain number of votes depending on the size of their states. You could think of the election as a bunch mini-elections in every state.

When a candidate wins the majority in a state, the state's votes go to that candidate. The candidate with the most electoral college votes at the end of the voting period wins.
-T.B.

Good luck: )