single sql, multiple schemas, one result set

Suppose a DB with dozens of schemas with same structure.
DB
Schema1
Table1
Table2
Schema2
Table1
Table2
Schema3
Table1
Table2
Then we want to execute a SQL on specific schemas and the result of it could
be a UNION ALL. So, how could be a function that runs that SQL on each
schema and results just on result set ?

Then I would use something like ...
select * from SQLFromMultipleShema(Array['Schema1,Schema3'],'select count(*)
from Table2');

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