tsql - Is this Correlated SubQuery Correct -

i trying wrap head around sub-queries (correlated). although book explains (ms sql server 2012) i'm still little confused. using 3 tables, orders, products , orderdetails want find sum of products shipped usa. came following query not quite understand how got there except correlation referencing 'where' in outer query. can correct work , offer better explanation book?

select p.productid, p.productname, sum(qty * od.unitprice) totalamount production.products p     join sales.orderdetails od     on p.productid = od.productid     n'usa' in        (select o.shipcountry         sales.orders o        join sales.orderdetails od        on o.orderid = od.orderid        shipcountry = n'usa')        group p.productid, p.productname; 

select     p.productid ,   p.productname ,   sum(qty * od.unitprice) [total_amount]         production.products productid join        sales.orderdetails od    on p.productid = od.product join     salesorders        o     on o.orderid = od.orderid     o.shipcountry in ('usa') group      p.productid,      p.productname 

the following should work - technically dont require sub query. bog down query.