I haven't used an alarm clock in 20 years and wake up within the same 5-10 minute window almost every day.

I have a friend that will just sleep through multiple alarms. But he can also just fall asleep in 10 minutes on an airplane and I can't sleep at all on a plane. He also takes lots of naps and I never take a nap unless I'm sick.

The worst feeling to me is waking up from some external noise. I just feel so groggy afterword. My body will feel better after 5 hours of sleep and I just wake up naturally compared to 5 hours, go back to sleep for 2 hours, and get woken up during the wrong time (middle of REM cycle?) and then I feel tired for the whole day even though I got more sleep.

Wasn't there supposed to be a smart watch out there you could set a window of time you wanted to wake up like "between 6:30 and 7:00" where it would use the sleep tracking to pick the optimal time to wake you up like at the end of a REM cycle? I feel like I must have hallucinated this because I'm wearing a smart watch (fitbit) now and no such feature exists.

Your friend could benefit from a sleep study. I'm not saying they have sleep apnea or UARS, but people with those disorders often have those symptoms.