How to stop someone from seeing your messages/location?

So it’s a long story but I’ll try to keep it short. My parents are currently getting a divorce. I know that my mom is able to see my dads text messages and his location from her phone. But I don’t know how.

I am over 18 I don’t want her seeing my messages too that’s personal stuff!! How do I know if she’s looking at my stuff too?? Like is there a way to check? in like settings or something if your stuff is being shared with another person? If so how to I turn off whatever setting that is?

And how is she doing this?

Our whole family has iPhones btw. If that helps idk.

I did try googling it but so many different answers about hacking and stuff and I don’t think that’s it at all my mom is not a techy person so I feel like this has to be something simple in order for her to pull it off.