Surprisingly the most effective treatment I've found for my own apnea is just wearing an inexpensive soft cervical collar to sleep, but there are lots of different things that can contribute to sleep apnea, so YMMV.

After dealing with unexplained ectopic heartbeats (sometimes PVCs, sometimes PACs) I saw a chain of doctors and that included both getting a sleep study done and seeing a cardiologist.

I was diagnosed with mild apnea (AHI just above 10), but despite the apnea being mild on the AHI scale, my Sp02 blood oxygen percentage was dropping into the low 80%s for extended periods per night down from a normal 95-99% most of the time.

I weigh 180 lbs at 6 foot 2, being overweight was a non-factor in my apnea.

Got a CPAP machine, it helped but even after dialing in my personally optimal pressure settings using the CPAP data with OSCAR my AHI was still kind of all over the place, from under 1 to as much as 6 seemingly randomly per night with no mechanical issues like mask leaks showing up.

Had heard (on reddit /r/CPAP) about some people using soft cervical collars to help with their apnea and I gave it a try and when wearing one my AHI drops down to 0.0 - 0.4 per night. The collar alone helps more consistently in my case than the CPAP machine does.

Basically my sleep apnea is almost entirely due to the fact that when I'm sleeping (even on my side, but worse on my back) I seem to naturally tuck my chin in toward my neck in a way that constricts my upper airway and the collar stops this from happening. This is very much YMMV, there are lots of different causes of apnea.