Do not store anything on your phone that you wouldn’t want your worst enemy to see. Do not even say it while your phone is nearby. Put your phone in an acoustic proof container/soundproof box if you need to have a talk with anyone. Don’t bother with airplane mode, it’s a sinkhole waiting to be discovered so assume it’s compromised until given sufficient evidence to the contrary.
With this in mind, you can sleep soundly knowing that your phone is a monitoring device, and as such privacy-related studies that demonstrate (as they always do) that your phone isn’t as private as you think will not come as a surprise and you will have no bad habits to undo/change.