If I buy a ubisoft game on epic games, can I use the ubisoft version of the trainer or do I use the epic games version of the trainer? (asking since ubisoft still uses it’s external launcher)
There is no guarantee that it will work. If there is only a trainer for ubisoft it is best to buy it there.
epic will have the same code as ubisoft for the game, and it also depends on the launcher, it may or may not call the native launcher, just experiment and you will get a good solid answer. Happy gaming.