While you would be able to target most iPhone and iPad users by targetting the respective device type mobile or tablet and the Safari browser, this segmentation would not apply to users using these iOS devices in combination with any other browsers.
There currently is not an option to specify if the mobile device uses iOS or Android (and potentially others) as their operating system. A use case for this type of segmentation or targetting is for example wanting to promote your mobile app available only on iOS.
An alternate solution would be to set a custom variable for users browsing your website on an iPhone or iPad and then target users with this custom variable set with an offer or template.
Below is the code that would need to be added to your Piano integration script. This can be done by going to Products → Composer → Integrate (button) → Edit Source. And adding the piece of code in the section with the tp.push functions.
iPhone
if (navigator.userAgent.search('iPhone') > 0) {\ntp.push(["setCustomVariable","iPhone","true"]);\n}
iPad
if (navigator.userAgent.search('iPad') > 0) {\ntp.push(["setCustomVariable","iPad","true"]);\n}
Or if you are using your own custom loading script, add the above snippets to your website's code where you've implemented the Piano script.
Then you would set up the User segmentation for the Custom Variables like this in order to target all iPhone users.
For example:
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