FaberCastell I have read the features page, but could probably do with a re read... I suppose I am trying to approach it with a greater understanding of the threats that it mitigates
For sure. I can also recommend the interview with one of the GrapheneOS developers on THe Hated One's channel.
https://www.youtube.com/watch?v=WkQ_OCzuLNg
FaberCastell Does this mean that even if you get a malicious app a reboot will spot it and get rid of it?
If a malware succeeds in getting persistence into the operating system it can not survive a reboot, because on reboot the bootloader checks the whole boot chain including the operating system's kernel cryptographically and fresh OS image gets loaded. https://source.android.com/docs/security/features/verifiedboot
FaberCastell What ways can a malicious app access sensitive data. Do you mean access your eg photos and other files, or can it force other apps to give up info re passwords or log in credentials etc
With Storage scopes the app can only access the files itself creates, even if it demand higher perissions. Storage Scopes tricks the app that it has full permissions.