It becomes part of the model code, but it may or may not be automatically adopted by a given jurisdiction, depending on the legislative language that the jurisdiction used in adopting the NEC.
A few examples that come to mind (not a legal expert, just what I would expect):
1) The law references the "2020 National Electrical Code as published by the NFPA on such and such a date." Subsequent TIAs are not automatically adopted. TIAs adopted before the specified date would be adopted.
2) Or maybe it says "the 2020 National Electrical Code including all TIAs and errata." TIAs are automatically adopted.
3) Or maybe the entire body of the 2020 NEC as of that point in time is literally incorporated into the body of the legislation. Subsequent TIAs are not automatically adopted.
4) Or maybe it just says "the 2020 National Electrical Code," and then it might end up being litigated whether that means the version that existed at the time the law was adopted, or whether the legislative body intended to give the NFPA the ability to change the law of the land in that jurisdiction via the TIA process.
Cheers, Wayne