"Under arrest," in this context, merely means Trump was in the custody of New York authorities during the time of his processing and arraignment. This was expected and planned.
On March 30, 2023, former U.S. President Donald Trump was indicted by the Manhattan District Attorney's office on charges related to hush-money payments made to adult film star Stormy Daniels, becoming the first ex-president to be indicted in a criminal investigation.
When news of the indictment went public, a spokesperson for the Manhattan court overseeing the case announced that Trump's arraignment would be held on April 4, 2023. As Snopes previously reported, representatives for Trump, the Secret Service, and New York Authorities negotiated, in advance, the terms of Trump's self-surrender.
As part of their agreement, Trump would voluntarily turn himself in to the court system, but would not be handcuffed. And as The New York Times reported, that is how the day played out: "While in custody, [Trump] was fingerprinted, but special accommodations were made for him: He spent only a short time in the office before his court appearance and most likely was not handcuffed nor was a mug shot likely taken."
Karen Friedman Agnifilo, a former chief assistant with the Manhattan district attorney's office, explained to Insider in late March 2023 that Trump would be "under arrest" from the moment he surrendered. That's because, technically at least, Trump was in the custody of the New York Court system from the moment he surrendered until a judge approved the conditions of Trump's release during his arraignment.
Because this sequence technically means Trump was "under arrest" for a period of time, we rate the claim as "True."