My understanding is that even when you turn your phone “off,” it’s not really off. This is anecdotal on my part. Could be wrong.
I cannot speak for all of companies, but this is not the cases for the companies that I worked for over the last 20 years. The ICs are designed with a simple connection between itself and the power button. The phone is not even in standby. What it is waiting for is a closed circuit which energizes the main IC, that in turn boots the phone. I worked for a Motorola and Qualcomm, and they didn’t do it because it would kill their battery life. Qualcomm is the primary chipset for 100s of millions of phones around the world. Others like, Apple, make their own chipset.