SQL Server Threads and Degree Of Parallelism
My original understanding is (likely incorrect) a scheduler may (when there is a need) use multiple workers/threads from the same NUMA node to process different tasks of the same execution request when it is a serial execution.


A serial plan will not use multiple threads it will only use single thread to perform the task. When the serial query would start it would be bound to a scheduler and that scheduler will not perform any other task unless it completes this query. The scheduler may wait for a process to complete thus forcing query to wait.

It is also worthy noting that MAXDOP setting controls the amount of thread used for parallel processing and MAXDOP is always specified per operator in the execution plan and not per execution plan

I strongly suggest you read below article by Paul White

Understanding and Using Parallelism in SQL Server