Responses
Well a isekai it’s a character that travel to other world in his body with his memories, a transmigration only travel the mind in the body of other character, maybe conserves the original memories or maybe not, in this case a Korean woman enters in the body of the empress and the original disappear (☠)
At first I am sure that Dongha was only isekai-d to Julia but after that chapter with Gaianna(?) I think she's really Julia and those memories she had after returning back time was her past life or smth?