Splitting SQL with unions alls is usually a matter of extracting each subquery into its own statement. You'll need to share your query for us to give more meaningful help.
But.
In general you don't want to.
If the subqueries are mutually exclusive and you're worried about performance, don't be. The database can optimize the query so it only runs the subqueries matching the input parameter:
https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:9538822000346932676 If maintainability is your issue, there are a couple of techniques you can use to help. One is to use the with clause to encapsulate your subqueries:
with complex1 as (
select big_query1 ...
), complex2 as (
select big_query2 ...
), ...
select *
from complex1
union all
select *
from complex2
...
Or you could stick the queries in "proper" views. And union-all those.