Steem is not mined. It was mine-able initially but the mining was stopped by Steemit Inc. All the Steem in circulation or supply is premined except a small percentage which is created annually through inflation (release of new tokens).

Can you help with my understanding please?

Steem is mined using Proof of Brain - through authoring & curating. But now it is not minable except for the new tokens, which is done by witnesses. In other words, the witnesses are the only miners - is this correct?