Best stretch for a tight lower back

Often times a tight lower back is actually a symptom or tight muscles around the back (glutes, hips and core) or a weakness in those same trouble areas (glutes, hips and core). Try stretches such as hip flexor stretch, pigeon pose, butterfly and frog stretch, and thread-the-needle. Sorry for the names, but look them up and try them out. If your glutes are really tight try stretching your hamstrings too; conversely, if your hip flexor is really tight try stretching your quads. There are reference online for how to see if you have a weak lower back – I'm not affiliated with them, but I know Athlean-X with Jeff Cavaliere has some very good videos on YouTube for this!
I hope this helps and good luck on your journey!