You, as people who know the difference between Linux and FreeBSD, what cell phone do you choose?
I happen to use an Android phone, to be exact a Pixel. More on that below. It has nothing to do with the difference between Linux and FreeBSD.
Is it an Apple because it's a BSD derivative?
The notion that iOS is a BSD derivative is nearly completely incorrect. It does include some components from BSD, but the bulk of the OS is Mach, and the user space and application layer is completely separate.
Or is it Linux because it's at least open source ...
BSD as at least as much open source as Linux is. Android is mostly not open source; only a very limited version of it has been released.
... and you can do what you want to do with it?
I very much hope that people CAN'T do what they want with the OS on my phone. I trust Apple and Google much more to deliver a functioning and secure system than I trust random volunteer developers.
I use Android Pixel phones.
Same here. I actually started using the Google-built Nexus phone many years ago, long before I became associated with Google. Which is funny, because at the time it was nearly exclusively used by Google employees.
But: In the last 10 years, I've been slowly becoming more unhappy with Android, and with Pixel. The user-inimical attitude of the development teams for the software is clearly visible: it is a phone built by SWEs for SWEs, and stability, quality and usability are secondary. Of my two close family members, one swears by their iPhone, the other grudgingly continues to use an Android Pixel, and I expect that an iPhone for me will be found under the Christmas tree.
Better integration between my daily driver laptop and desktop (all MacOS) and the phone is not terribly important; there the only remaining real hassle is having two separate contact lists.
I am impressed by how resistant IOS is against rooting. Wouldn't have thought that is would be possible to keep people out to this degree.
That's the other highly important factor in choosing a portable device. By construction, it travels in public with me, and can relatively easily get lost or stolen. Given how vital the phone is for 2FA, that might be disastrous. That's why one also has to be careful in choosing a carrier (not just phone hardware/software provider) that is very responsive to requests for locking, and has good fraud/theft detection systems.
