Second, is that even admissible without a warrant?
Given the way the expert witness is described, and the likely course of events that day, I don't think they'll argue that the cell-phone evidence was proof of some sort of insurrection as much as they want to show Trump as an aloof social-media addict who was looking up memes and tweeting while congress burned (to paraphrase Tacitus).
They wouldn't have entered this into evidence if they didn't have that warrant, and considering how the DoJ has pursued and persecuted nonviolent protestors for years after the fact, and how DC judges and juries have been convicting them to the hilt, it's pretty easy to believe they found a lawyer in the DoJ to go before a DC judge to approve whatever warrant they needed.
Read this!
https://www.politico.com/news/2023/12/11/special-counsel-trump-phone-data-trial-00131196
Special counsel reveals plans to use Trump’s phone data at trial
Jack Smith indicated that he plans to call an expert witness who extracted and reviewed data copied from Trump’s phone.
Special counsel Jack Smith has extracted data from the cell phone Donald Trump used while in the White House and plans to present evidence of his findings to a Washington, D.C. jury to demonstrate how Trump used the phone in the weeks during which he attempted to subvert the 2020 election.
In a court filing Monday, Smith indicated that he plans to call an expert witness who extracted and reviewed data copied from Trump’s phone, as well as a phone used by another unidentified individual in Trump’s orbit.