How does a B-tree work? How would you use a B-tree to implement a
secondary index(in a database)?

Here is 2 tutorials on basics of b-trees
http://cis.stvincent.edu/carlsond/swdesign/btree/btree.html
http://www.bluerwhite.org/btree/

As for it's use in a database, a b-tree is often used to store an
index on disc when you have enough entries that they cannot be loaded
at once into the memory of the machine, i.e into RAM. By using this
structure, it's possible to increase significantly the speed of a
search on disk and then load required entries into memory. This
process is often automated when you are using a database system such
as MySQL or Oracle.

You haven't answer the quenstion How would you use a B-tree to implement a
secondary index(in a database)?

You should n't be paid the $10.  You should reserach the question and
post an answer that answers the question.

Ignore the previous comment. I thought it was an aswer to the quesitons.