SQLite is an "in process" library -- it executes entirely "in line" within the callers thread.
The only possible exception to this is that if-and-only-if all the following conditions are met:
- the library code was *not* compiled with SQLITE_THREADSAFE=0
- the library code was compiled with SQLITE_MAX_WORKER_THREADS greater than 1
- the current configuration is not SQLITE_CONFIG_SINGLETHREAD
- the value returned by PRAGMA THREADS is greater than 1
(initially set to min(SQLITE_MAX_WORKER_THREADS, SQLITE_DEFAULT_WORKER_THREADS)
then SORT operations may utilize additional SQLite3 created threads up to the limit set by PRAGMA THREADS, such as for the creation of indexes or the operation of ORDER BY or GROUP BY sorting.
Otherwise, all operations are carried out linearly in the single thread of execution provided by the caller.