This article sums it up pretty well:
The first Trump trial is here, set to begin on Monday, with the allegation being that the former president violated New York law by having his company misreport payments to Stormy Daniels as “legal fees” rather than campaign expenditures.
...
Misreporting business expenses is normally, at most, a misdemeanor. Bragg seeks to ratchet it up to a felony here by arguing that the misreporting was done to cover up a crime. That alleged crime is a violation of the Federal Election Campaign Act (FECA). The theory is that Trump’s payments to Daniels were campaign expenditures and thus needed to be publicly reported as such. By not reporting the expenditure, the theory goes, Trump prevented the public from knowing information that might have influenced their votes.
There is one big problem with this theory: The payments to Daniels were not campaign payments.
Commentary on the case has consisted of accusations against Bragg and Merchan, but Trump has a strong defense on the merits.
thefederalist.com
So in a nutshell, Trump was found guilty of misreporting business expenses that the prosecution successfully convinced a jury was done to cover up a bigger campaign finance crime.
However, as the article concludes, "Had Trump made these payments with campaign funds, it seems a near certainty he would now be facing criminal charges for a knowing and willful diversion of campaign funds to pay personal obligations." You're just supposed to forget that "The case surrounding Trump’s payments is one that
both the Justice Department and FEC rejected to prosecute in recent years. The Justice Department in 2019 "effectively concluded" its investigation into Trump’s payments. While in 2021, the Federal Elections Commission announced that it had dropped a case looking into whether Trump had violated election laws for the payment to Daniels." (
source)
So since they couldn't get him on campaign finance law violations, they got him on misreporting business expenses. Because they had to get him on
something, amirite
?!