(should they focus on it they would not get access to its internals)
If they have the device
and you, you may be forced to share it depending on the law of the country. Some countries have gone mad already:
Section 49 of the Regulation of Investigatory Powers Act 2000 contains the relevant power.
If your phone has been seized, or in circumstances where they have the power to inspect it, the police can give you notice that they require you to provide the PIN or “encryption key” to allow them access. The same applies to other devices such as computers.
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If you do not comply with a properly given notice, you can be prosecuted. If you know the information required and refuse to provide it, you can be sentenced to a maximum of 2 years imprisonment or 5 years imprisonment for an offence involving national security or child indecency.
Good luck proving you forgot the key.