Steem APR - How is this determined, exactly?

I've been here almost two weeks and I've never had any APR percentage change for my STEEM Dollars. How is the APR determined for Steem Dollars?

I'm wondering how the APR is determined?
Who sets the APR's? The Witnesses?
Is it determined by how long you hold the funds in the wallet?
Is putting the STEEM Dollars in the Savings going to affect the APR percentage?
Does the APR coincide with its USD value and only gets APR if above $1 USD?
I would assume the APR would decrease as the SBD value climbs relative to the value in USD as well, correct me if I'm wrong, please.

Thank you,
Pfiffel (Niko)