ARM Core and UI

Dear Osmocom members, I am new in OsmocomBB but trying to understand how all the stuff works. And I have a question. As I understood we have DigitalBB Calypso in our Motorola phones which consist of DSP and ARM Cores. If we are talking about "normal" phone firmware, In DSP L1 processing is done, in ARM core L2 and L3 processing of GSM stack is done. But we must also have some "regular" OS somewhere, am I right? So I mean all the menus, applications like "Settings", "Phone book", etc. In modern phones, I believe Adnroid or iOS communicate with some RTOS running in ARM core of DigitalBB. But what is about Motorola phone? Does it have separate ARM processor/RAM for a refular firmware? Did you create some applications for ARM part of DigitalBB and some applications for UI which run outside the RTOS? It's possible that I am confusing with all the terms but I would appreciate if you help me to understand. Kind regards, Anton