How does one write & run queries against a database/warehouse if they have no knowledge of how the data is structured?
Short answer - you can't.
It's like asking how you can do exploratory surgery on someone, but you know nothing about Anatomy.
OK, you can do slightly less damage in a database, but the principle is the same.
You have to find out what's in there before you can [usefully] start working with it.
How to find that depends on the DBMS but, luckily, most of them support standardised metadata, held in tables within "information_schema".
select * from information_schema.tables;
select * from information_schema.columns;
.. and many others.
If you're really lucky, there will be constraints between the tables - these relationships can be priceless in working out how one thing relates to another. There are even tools that will pull this information and graph out the tables and how they join together.