You can almost think of memory palaces more as maps or pointers than the data structures themselves. You're piggybacking off of existing neural connections in your memory circuits and placing new connections in between or at the ends, and activating those new synaptic connections by activating the ones you're already familiar with which are connected to them.

The reason we use memory palaces specifically is because our visual memory is incredibly strong. Many techniques take this a step further and add an element of time, allowing you to make use of your chronological memory as well, filling in the gaps between "scenes". It works exceedingly well, I can memorize a pack of cards in about 2-3 minutes with this technique.