Bottom line is *every* operating system will have vulnerabilities discovered and always will. Some known, some unknown. The known ones get patched, the unknown ones eventually will once discovered.
The whole "secret backdoor" thing...ugh. Too many conspiracy theorists. If there *are* any secret backdoors, then they must be reserved for secret national security operations, because I can speak from personal experience they are not available to local law enforcement or run of the mill criminal cases.
If your OS on your modern device is fully patched and up to date, doesn't matter if it is Apple, Microsoft, Google or Linux, you're going to be very secure from prying eyes as long as you are using strong passwords, PINs, etc. You want into a fully patched modern device? It's a waiting game for vulnerabilities to be disclosed, as those newly discovered vulnerabilities can then be used to compromise the device. Just hold onto the device, powered down with no access to the internet for updating, and bide your time until a way in is discovered (and eventually a way in *will* be discovered). If it's critical evidence, file whatever paperwork you need to keep the statute of limitations from expiring.
Also....never sideload apps/programs and you'll be much safer.
Here's an example of Google's increased focus on security:
https://9to5google.com/2023/01/23/android-14-block-install-outdated-apps/