If your relatives are significantly tech illiterate, I'd skip the smartphone entirely and go for a locked-down Linux desktop + feature phone. The most dangerous apps are big legitimate ones.
If you do go for a smartphone, my experience tells me that there's no difference between Android and iOS. The biggest sources for shady apps are the Google Play Store and Apple App Store. Shady stuff on the web can be easily defeated using an adblocking browser, which is essential for older relatives.
Sir a Linux desktop recommendation for non-tech-literate elders is just silly
It's getting less silly every month! So many people in that boat only use the web browser anyway.
With a well-supported hardware configuration and a working web browser, even a non-techie may have a more stable experience than they would with Windows.
That has as much to do with the decline of Windows as with the ascent of desktop Linux, but still.
Why? As long as you use a regular desktop distro, it's simple, pleasant and easy to use. This was already true 10+ years ago.
It isnt silly. We did this with my 92 yo grandma back in the 2010s. She had a browser, an email client, word processor, printer.
Bulletproof, immune to all the apps and malware. She couldnt break it.