Parallel Query Block Of Union All Set

We have very large table having data more than 1000 millions rows. We divide this table into four physical tables say A, B, C and D. The physical horizontal partition of data of this original table is done based upon their business policy.

Each partitioned table has contained data of particular business entity. Further each table has partition and sub partitions based upon business rule.

We have to retrieve data from all these tables as follows:

select a1, a2, a3, a4, a5, a6
from A
where < logical filter condition>
union all
select b1, b2, b3, b4, b5, b6
[code].....

We observed that above each query block execute in serial one after another and individual each query block capable to process data in parallel from respective table.

How does this above query able to execute each query block in parallel?