In general, people hold their phones with one hand and operate them through thumbs. And, most of the interactions are thumb-driven. So, it is safe to say that they do not forget to consider the positions in which users hold their phones. In users, the thumb movement is one of the most common ones when it comes to holding their smartphones.
This is the best tip: “Give a Thought Reachable Thumb Zones”. I do not agree with the onboarding tip. Apps should have as little onboarding as possible.