Some operations in SQL Server will turn your entire query plan serial (single-threaded), others will just reserve a so-called “serial zone”. I read up on this stuff a number of years ago (inc...
https://sqlsunday.com/2018/08/09/parallel-and-serial-operators/
The subject of parallel execution of SQL Server queries is at times somewhat shrouded in mystery and uncertainty. Since the concept of parallel execution is such a significant (and indispensable ...