In the Stellar whitepaper, there are examples of various Quorum structures. Which one does Stellar use? Is there a visual representation that I should look at?
In short, there is no fixed quorum structure required. Fig. 3 and 4 in the whitepaper are just examples showing how flexible Stellar's quorum structure can be.
For "Tiered quorum structure", it is more related to an individual Stellar Core node. For "Cyclic quorum structure", it is more related to the big picture of how Stellar Core nodes are linked up. The two structures are not mutually exclusive. You may refer to the example config to take reference on the setup of Tiered structure.
No particular visual aid for you, but I would recommend you to forget about this comic if you have ever read it (for that it can be rather ambigious)...
You can find more information from this link page. https://www.stellar.org/developers/guides/concepts/scp.html