Well you should subtract out the kids. I'm not going to get irritated about someone under the age of 18 not voting. That's why it should be eligible voters, not Americans.

33%, but yeah. Both are true.

66% of eligible are voting, 33% of eligible are not voting. Of the 66% that did vote, half of those (I used 49%) voted for Trump. Half of the 66% is 33%.

So, of the eligible voters, 33% voted Biden, 33% voted Trump, and 33% didn't vote.

Or, if you want math:

.66 * .49 = .32 or 32%

Edit: I misunderstood the question and gave an off-topic answer. Someone else commented the correct reason: not all Americans are eligible to vote. Children, felons in certain States, etc. Since they didn't/can't vote they're not counted in the "half of America" statistic often quoted.

Half of eligible voters voted for trump, which is about 32% of America in 2020.