On closer look, it's a case of needing to aggregate before the join, not afterwards.

SELECT OPP.ID,
   OPP.NAME,
   OPP.AMOUNT,
   ...

What's often helpful to debug something like this is to take the aggregation off, and look at the full resultset.

SELECT OPP.ID,
 OPP.NAME,
 OPP.AMOUNT,
 OPP.ATRC1,
 OPP.ATRC2,
 OPP.TRC1,

Ok Jeff, I'll interrupt my Friday evening... 🙂

There's two main reasons to split up a database into multiple filegroups.
1) Performance
2) Restore times/recoverability.

Splitting for performance...

I wonder if it was malware, or if someone intentionally installed it on the server to make some profit on the side from the cryptocurrency mining.

In general, there's nothing wrong with derived tables, in views or elsewhere.
My general rule is to write any query in the simplest, most obvious way, and tweak it for...

Or condolences, whiskey and a shoulder to...