iPhone shouldn't even be on this list. Its a nonstarter straight out of the gate when it comes to installing a custom ROM. Whereas Android (the operating system) is open source (AOSP or Android Open Source Project). If you have an open source iOS repo you can show me where you can modify the spyware out of it and harden, maybe it can be considered, but in its current state, its a nonstarter.

So of the devices that can run a spyware removed version of Android, why Pixel? Because Pixel is the only hardware that allows users to lock the bootloader after they have unlocked it to load a custom OS. That's it. That's the reason. If you have other hardware that can run Android that allows you to do that, it should be taken into consideration.

Now all of that said, Linux phones that are not Android (Android is a fork of Linux and Linux is a fork of Unix) do exist and if you feel so inclined, you should look into them.

Pixels are essentially the reference/signature device for all other Android devices, since Google are the people behind Android after all. Pixels get anything to do with Android first before other OEMs, such as version upgrades (13 -> 14) among other things. Other OEMs like Samsung are in most cases the same as Pixels in security but Samsung are unnecessarily protective, so their security hardware permanently deactivates once you install another OS.

The main interest in Google is to make the best Android that takes advantage of all of it's capabilities, stock Android also takes advantage of the hardware the Pixel has. Developers of apps AND operating systems also rely on the Pixels as development kits, as they can develop software designed to take advantage of hardware security modules since they are the only ones that allow you to use them in a third-party operating system and also allow you to install other operating systems safely and securely.

Backdoors would be noticed and publicly known if they were to exist. There are a lot more reasons to attempt to disclose or discover it then to cover it up. Academic researchers, anti-Google companies/competitors, foreign countries hostile to Google/the US, forensics companies, spyware companies, hackers and paranoid individuals would have a field trip if they existed. Many people talk about backdoors but fail to look at the bigger picture that it's not just the USA interested in it. For example, Chinese actors have been attributed to implant malicious UEFI motherboard firmware to their targets. You could suggest the only reason we are able to know that is because intelligence against China want to publicise their exploits so that their exploits become useless. It would be in the same interests of nations hostile to yours to reveal that.

Just because a system isn't open doesn't mean that it can't be monitored, reverse engineered or modified. Ordinary people can and do attack hardware, like people exploiting Trezor to bruteforce the wallet, or Ledger's Donjon team assessing hardware of their competitors.

It is not impossible that there is a backdoor in Pixel devices. None has been found. One difference between Google and Intel is that Google doesn't need a backdoor for the vast majority of people. They just use Google products like Google Andriod, Chrome and Gmail.

Researchers can monitor the network activity and radio activity of these phones and have yet to find a back door in Pixels. There are so many privacy and security issues that Graphene fixes but it is still a phone. You should research it for yourself. In the end you will have to choose who to trust. The most private option is no phone.

"GrapheneOS is an open source project with an open development process.The GrapheneOS sources are hosted in the GrapheneOS organization on GitHub"