You're almost certainly reading that correctly, that pMem is a pointer to a pointer to a block of memory, plus some metadata for that block of memory, and sqlit

You're almost certainly reading that correctly, that pMem is a pointer to a pointer to a block of memory, plus some metadata for that block of memory, and sqlite3VdbeMemRelease either releases the memory back to the application-level pool, frees it back to the memory allocator (glibc malloc/jremalloc/tcmalloc/dmaloc/etc.), or directly brk()/mummap()s it directly back to the OS. I have no experience with sqlite internals, but it's pretty clearly doing some application-level accounting on top of the normal memory management.