Though there is no any built-in feature to sync between iOS and Android devices, you can still sync Reminders and lists using some apps. Thanks to that, people can access the same page regardless of different operating systems.
Google Calendar is obviously the best choice to sync those apps between two platforms. When you use Google Assistant on your Android device, the reminder will be updated to the Google Calendar in Reminders calendar section. Additionally, even without the help of Google Assistant which means you use Google Calendar only, it is still the great method to follow your to-do list.
So the steps is pretty simple. You just need to download the Google Calendar app for iOS, sign in with your Google account and then things are done.
However, this method will not work for Siri unless you go the same way. To be specific, if you use an Android device and want to sync reminders from iPhone, it’s unable to do sync your to-dos from the Reminders application. You are still allowed to sync over your Calendar app events by following to URL of your calendar in Google Calendar. However, this seems a bit inconvenient as it takes up to 24 hours for events to appear in your Google Calendar.
As for Apple, it does not enable users to sync Reminders with other accounts. Otherwise, with Google, your options are much more opened.
Therefore, in case you need to sync reminders and calendars across the two platforms, simply use Google Calendar app for all. One small problem is that you may confuse with your sync settings but that’s unavoidable.
Besides, to help with syncing, users can also use some other Android apps. However, those are most unlikely to successful and some of the well-known ones, such as Smoothsync, haven’t been updated for years.