She "gave written notice" to California Governor Newsom, with her resignation effective today (1/18/2021).  Newsom had previously announced the appointment of Alex Padilla to fill that vacated Senate seat & finish out the balance of her term, which ends in 2022.

 
The R's made a big stink on here wondering why she hadn't done so immediately after the election.  Talk was, she hadn't because she knew that Trumpolini & Dense had actually won; therefore, she wasn't going to leave the Senate after all.  They also thought she must give at least two weeks notice before leaving.  All total nonsense, of course.  

 
Let reality set in.