He informs her that her uncle, Jeor made Jon his steward and chose him as his successor because both of them understood that the real war wasn't between a few houses but between the living and the Dead.

Source: https://gamerant.com/bella-ramsey-game-of-thrones-role-explained/

When he arrives at Castle Black, the Night's Watch doesn't even have a Lord Commander, as the Wildling crisis is at its height after Jeor Mormont's () death, so Ser Alliser Thorne () is acting Lord Commander.

Source: https://collider.com/game-of-thrones-book-change-jon-snow-death/