How do you warmup your lower back for leg session or back session to avoid lower back injuries?

I don't. Proper bracing and doing warmup sets of whatever exercise I'm planning on doing has always been enough, although I usually don't lift very heavy. Minimum of 8 reps on squats for example.

If I were to have lower back problems I would simply look at different exercise examples on youtube and try them out. Different exercises will work for different folk, there is no best exercise for waming up the lower back. I would also try lifting even lighter.