Hi Michael,
In general, Scrum has increments whereas kanban is continuous. In either system the backlog should be prioritised, or at least have enough prioritised backlog items to be selected.
In sprint planning the team creates a Sprint Backlog from the Product Backlog and then through the sprint team members can select any of those items until the sprint ends. Often in kanban you would pick from the top of the prioritised backlog .
If the team is self organising well they’ve probably had enough conversations with the PO and team to know which item to pick up if, for example, they run out of sprint backlog items.
In short- the next item selected should always be the ‘right’ one through whatever system the team uses.