Ending the turn this way means the following things happen in order: (1) All spells and abilities on the stack are exiled. This includes any abilities that triggered as a result of destroying all artifacts and creatures as well as Ultima, though it will continue to resolve. It also includes spells and abilities that can't be countered. (2) All attacking and blocking creatures are removed from combat. (3) State-based actions are checked. No player gets priority, and no triggered abilities are put onto the stack. (4) The current phase and/or step ends. The game skips straight to the cleanup step. The cleanup step happens in its entirety.

If any triggered abilities do trigger during the process of ending the turn, they're put onto the stack during the cleanup step. If this happens, players will have a chance to cast spells and activate abilities, then there will be another cleanup step before the turn finally ends.