Ending the turn this way means the following things happen in order: 1) All spells and abilities on the stack are exiled. This includes spells and abilities that can't be countered. 2) If there are any attacking and blocking creatures, they're 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. 5) The cleanup step happens in its entirety.

If the turn ends during the end step after "At the beginning of the next end step" abilities have triggered but before they've resolved, those abilities won't trigger again next turn.

If the turn ends before the end step, any "At the beginning of the next end step" triggered abilities won't get the chance to trigger that turn because the end step has been skipped. Those abilities will trigger next turn at the beginning of the end step.

If any triggered abilities do trigger during this process, 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 is over.