Others have already given you an answer, but for future reference, whenever you need to find something out about star trek there's memory alpha. It's basically the Trek encylopedia and is very very very detailed. So in this case:

The alternate reality was a new reality created on Friday, January 4, 2233 (stardate 2233.04) when a temporal incursion caused by time travel of the Narada, a Romulan mining vessel from the year 2387, disrupted the time continuum of the prime universe. Accidentally traveling back to that point in time, Nero, the Narada's captain, attacked the USS Kelvin resulting in the deaths of several crew members, including George Kirk and Richard Robau, and the destruction of the Kelvin itself. Spock arrived to the alternate reality in 2258 and was captured by Nero, who used red matter to destroy Vulcan. However, Nero's attacks united the crew of the USS Enterprise, who foiled his attempt to destroy Earth. ... Scans and telemetry of the 24th century Narada, taken by the Kelvin, were brought back to Starfleet by the survivors on the Kelvin's shuttles. Therefore, Starfleet's development and construction plans were slightly altered, making everything potentially more advanced, slightly ahead of schedule. ... co-writer Simon Pegg had a different view of how the alternate reality diverges from the prime timeline from Orci and Kurtzman, believing events before 2233 were different too ... "Spock's incursion from the Prime Universe created a multidimensional reality shift. The rift in space/time created an entirely new reality in all directions, top to bottom, from the Big Bang to the end of everything. As such this reality was, is and always will be subtly different from the Prime Universe."

https://memory-alpha.fandom.com/wiki/Alternate_reality