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.