I think 90% chance of apple going for the EU-Specific model just like they did for Digital markets act, 10 chance of screws, 0% chance of actually popping the back cover off with bare hands.
For Samsung, 50% chance EU-Specific models 40% chance screws, 10% chance back cover tool-less removal. (Edit: typo)
As for other smaller manufacturers, probably just 50%/50% either screws or tool-less back cover removal.
While the phone not turning on can indicate other issues, sometimes it’s just the battery.
Thought it was dankpods, but it was his mechanic James:
https://youtu.be/IpT7h8VJX-0
It’s generally not just the battery though. A battery going completely dead in a recent mobile phone and not holding any charge whatsoever or even functioning is extremely rare.