They have two modes: 1. Transcribed tabs. They own the transcription as they have done with their catalogue in existing Rocksmith games. I believe they claim they use some “AI assistance” here, but I could be wrong. At the end of the day, they pay a large number of musicians to chew through songs and write tabs. 2. Chord following. They use automated transcription to get the chords for a very large number of songs. This is a new mode in the newest game and is entirely automated, but limited.

With OG Rocksmith (and 2014), the charts were supposedly all done by the folks at Ubisoft. They'd watch live videos and have multiple go-overs with each other to make it as accurate as possible. Besides, "official" tabs (at least historically) were usually not done by the actual artists, but some company that was just given the recordings and paid to figure it out.