As of 2019, the population of Wisconsin is 5.822 million. Of that, 4.53 million are of voting age. According to latest data, 1.63 million voted for Biden, and 1,61 million voted for Trump, for a total of about 3.24 million votes.
The voting rate is then 71.5%.
I don’t know where you get the 90% voting percentage. It’s actually lower this year than in 2004, for example.