For night landscapes in truly dark conditions, autofocus often won’t work, so manual focus is the usual solution.

Useful approaches:

• In daylight, focus on a very distant object and mark that lens position with tape as your infinity reference.
• At night, start near infinity, take a test shot, review focus, and fine-tune. To speed this up, use a high ISO for focus tests, then lower ISO for the final exposures.
• If focusing on stars, use the brightest star available and, if possible, a magnifying finder or focusing aid such as a Hartmann or Bahtinov mask.
• If you need to focus on a closer foreground object, a small laser pointer aimed at the subject can create a visible point to focus on; switch it off before shooting.

One caution: the hard stop or infinity mark on many lenses is not always perfectly accurate, especially on modern lenses, so don’t assume simply turning to infinity will guarantee sharp stars. Test and verify.