I use an alarm.
Of course, the answer isn't as simple as using any alarm app on your phone or even a physical ringing clock. It really boils down to what kind of sleeper you are.
There's people who can sleep through airplane turbulence, a crying baby and a stressed out couple sitting next to them on a flight. I have massive respect towards those people. Albeit it's a pretty cool ability to have, it isn't all fun and games. (Major oversimplification incoming) The root reason behind why some people seemingly never wake up, regardless of how hard they try, is simply because of stress and past trauma. Being raised in an environment full of noise even during "calm" hours never lets the brain go into the rest & digest mode. When these people eventually fall asleep, their brain makes the most of this calm time, keeping them asleep for as long as possible.
I'm thankfully not that kind of person. However, what I lack in sleepiness, I make up for in terms of persistence. To be more exact, after my alarm rings and I snooze it... I go right back to sleep. What can you do for a person that can wake up, but doesn't stay up after the alarm stops ringing.
My solution
I explored many solutions. I tried using my watch, which has a little vibration motor that can work as an alarm. Of course, a little vibrating doodad on my wrist ain't waking or keeping me up whatsoever.
I tried using duct tape freshly prepared on my (hairy) thigh the day before. I then ripped that tape right off after my alarm rang. It worked, but I'd consider it a rather cruel method of keeping myself up. Thinking back at it, I also realized I'd run out of hairs to remove. Making the method useless for a couple of months - at least before new hairs starts growing.
So then, I thought of something in between. Ring the obnoxious alarm, slowly scale it in volume (get my PTSD going before it's at full volume) and make it so that I have to scan 1 (or more) NFC tag(s) for it to stop ringing. No snoozing, no emergency stops - just pure hell.
I found this lovely app called NFC Alarm Clock. It's FOSS and has no ads! You add your NFC tags and you're essentially done. I happened to have a few NFC tags made out of hard plastic circles that I "strategically" placed around my room. One where I'd have to squat to reach it, another where I'd have to go on my tippy toes to scan it and another at my height - like a free one to get my day going. The app randomizes the order in which these have to be scanned, so I can't just wake up and automate the same thing over and over - defeating the whole purpose.
I reckon you don't even have to use blank NFC tags, you could theoretically use an RFID tag that you use to check into your workplace or a car key with RFID functionality.
After I scan all 3 tags, the alarm stops ringing and I'm pretty much woken up by that point. It's still a bit of a cruel way to wake up - jumping out of bed immediately from a resting heart rate probably won't be good for the blood pressure, but I can stay up without causing miserable pain to myself!