As it doesn’t matter to this answer, I will here take your word that whatever happened was actually cheating. I will also ignore the nature of the contents you want to hide – it may as well be embarrassing fan fiction.
What will likely happen
For whatever it’s worth, I was involved in some exam-board hearings on cheating incidents in Germany as an examiner during the pandemic.
The exam board needs to collect evidence and you already gave them some. Now you can easily forge screenshots and protocols, while showing the respective content within the interface of the software is much more difficult. Therefore, the exam board may want to see this. However, to this end they do not need to use your phone themselves (and should not), but ask you to show the respective contents to them on your phone. They may ask you to navigate to certain points of interest or similar. Mind that all of this is them asking you to volunteer evidence (against others), not forcing you. In particular, if you do not want to show something, you can always refuse, with privacy being an excuse that should not raise any suspicion.
Mind that the exam board probably considers you on their side here and has no reason to suspect you, in particular since you failed the exam.
The worst that can happen
Cheating in exams is a minor offence (Ordnungswidrigkeit), see, e.g., § 63(5) of NRW’s university law.
This usually only gets invoked in large-scale organised cheating or commercial cheating.
Moreover, cheaters may escalate against the exam board’s decision to a court of law.
In such a case the court or investigators may ask you the same things as described above, and you can refuse to do certain things on the same reasons.
In particular note that as a witness you have a privilege to refuse evidence (Zeugnisverweigerungsrecht) if you would be incriminating yourself or damaging your own honour.
Even if they seize your phone (which I highly doubt), they cannot force you to unlock it. For example, see this opinion by a lawyer, which states that this cannot be done even for grave criminal offences.
A note on suspicions
One of the things you apparently want to avoid are suspicions. There is no legal protection against those, after all it’s just a state of mind. However, given your depiction, I would not expect that much suspicion has fallen on you, and the only way it would is if you suddenly retract all cooperation with the exam board.