They’re the most secure because they have the most exploit mitigations, best sandboxing, and are the most up-to-date, and Edge supports MDAG on Windows and has a hardened security mode which disables JIT and enables more exploit mitigations.
Google Pixel phones properly support verified boot for third-party operating systems and they are the only phones that use the custom Titan M2 chip. No other phone comes close to being as secure. This goes to show that sometimes DeGoogling and alternatives can be harmful to privacy.
Most custom ROMs are known to break the Android security model. A lot of ROMs do not support Verified boot and most require rooting the device or leaving the bootloader unlocked. There is a reason mobile network operators do not allow unlocking the bootloader: to protect their customers.