Many of us don't get the sleep we need. What techniques, habits, or apps have you found that help you increase sleep duration and quality?

Every person is different, so some things that helped me might help you.
I too had problems with my sleep. Mainly because of stress at work. Add to that some personal issues, and you got yourself many sleepless nights.

My sleep quality improved when I started practicing mindfulness meditation before bed. There are plenty of free guided meditations on YouTube or mobile apps that you can try out. My personal favorite is the Calm app.
And regulating my screen time an hour before sleep has also been effective. I use a blue light filtering app, which eases my eyes.

And then there's pillows and mattresses. You'd be surprised by how important they are for your sleep. If they are comfortable and good enough, you'll want to go to sleep ASAP. I use the Pluto pillow from https://plutopillow.com/. Very comfy, and the price is reasonable. And the mattress is from Tempur-Pedic. It's a bit pricey, but I'm very satisfied with it.

And a workout routine and a balanced diet have been crucial to my sleep improvement. I exercise daily (I use an app that shows what and how much to do) and eat three times a day. No sugar an hour before bed.
I hope these tips help you as they've helped me.

Struggling myself right now. REM restriction gets me back on track.
Room must be cool and dark, sunshine early in the morning seems to help too. I have to be physically tired so I work out daily

I think the answer is different. I normally sleep better if I do a lot of heavy work outside. The work makes me tired, being outside calms my soul.