Xtreix It would be interesting to have more information about this “Bittium Secure OS” and their “Hardened Android OS.”
Obviously i'm just guessing here, but i guess it is just dual boot, and perhaps their 'Bittium Secure OS' integrates with some private 'military' infrastructure(private 5g bands, ad-hoc radio plarform etc...) & software, while their 'Hardened Android OS' perhaps is to be more used as a regular phone and makes use of public cellular infrastructure, maybe their hardened android os might even use some 'parts' of grapheneos like their hardened malloc etc. Though this is all speculation on my part.