Yoga in general and deadlifts are what I do. Back extension in both directions laying on your belly on a bench is good for strengthening it safely, start with small range of motion and extend it as you can.

Remember a few things - we spend more time bent forward than back, so make sure to strengthen the back in the arched direction as well. The whole body is connected so develop it in a balanced way. And move in every direction (intentionally). That last one is what yoga is so good for.

I’m on the yoga train. Specifically downward dog into a bunch of cat/cow’s mixed in with other stuff.

Tai Chi for staying limber

Sun salutations every morning.

It depends on the cause of your back pain so I agree with the people who said maybe see a doctor. Some people have weak back muscles so strengthening exercises help. My back pain was caused by tight hamstrings and overuse of my back, so I fixed it with a lot of hamstring stretches and getting out of my office chair as much as possible. My brother in law has a bulging disc so neither of those things would help him.

Probably the biggest help for me was WFH so I could get out of my office chair - I can lie down, walk around, or sit in different chairs when I’m taking a break, and I take a lot of breaks. I stretch my hamstrings after most workouts so I’m warmed up. I bend over to touch my toes with my feet together for 90 seconds, starting gently, breathing as I relax, and slowly increasing the stretch a little as my muscles loosen up. Then I take a 30 second break, then I move my feet to somewhere width apart and do another 90 second slow hamstring stretch. Another 30-second break, then I put my feet about halfway to a split and do a other 90 seconds touching the ground. Then a break then as wide as I can go and bend over to touch the ground. I think the slow process really helps me relax.