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.
If anything like how they were before - dead easy. I had one for my old HTC Hero that was just a chunky ass battery with a bigger back to suit. Wouldn’t put it past a manufacturer now to make it difficult on purpose though.
You can get ‘charging cases’ that are loaded with an extra battery, cover the phone, and plug into its charging port as a workaround for sealed devices. There’s usually a button that switches the case on and starts charging the internal battery from the external one.