One of the trickiest things to understands is how memory works for hosts. Prior to Ford adding Arnold’s reveries, the hosts essentially only had short term memory. They would normally be wiped during maintenance cycles and set back on their loops.
After the reveries are added, hosts are able to access memories even after a wipe. They start experiencing difficulty with memory when they can’t distinguish it from reality. Felix tells Maeve that memory is different for hosts (S1 Ep8 10m). Humans have imperfect and fuzzy recall. Because the hosts have perfect recall, memories feel like reliving the event.
As the hosts experience further degradation and fragmentation, some of them learn to control their memories, and can interact with them like a lucid dream. Dolores, Maeve, and Bernard all exhibit control over their memories.