Hey there, Sef1975.
Welcome to the Apple Support Communities! Thanks for posting with questions about Airplane Mode, device security, password resetting, and Family Sharing. We'd be glad to provide you with some info on these topics.
To get started with, could you provide a little more insight into your question about Siri? Airplane Mode is where your iPhone or other device is used where cellular data and Wi-Fi are both turned off. Many functions of Siri require a form of internet access to work properly, so we'd like to verify exactly what you're trying to accomplish by putting Siri into Airplane Mode.
For your second question. iPhones are extremely secure, so accessing one from another device isn't something you should likely worry about. If that is something that you're worried about, read over Use the built-in security and privacy protections of iPhone for some information on device security.
You also mentioned that you were having some trouble with your Apple ID password. If you'd like to work on resetting that password, review the information in If you forgot your Apple ID password for those specific steps.
Your final question was What is Family Sharing? Check out the slew of information on that page as it details many of the great features of Family Sharing and what it can do for you and your Family Sharing group.
If you need more insight on one of the above pieces of information, be sure to let us know.
Take care.