Apple has made a significant effort to increase the security of your data, without negatively affecting your ease of use of the gadget. For this reason, iPhone default settings may not be perfect for everyone. 
The moment someone else picks up your phone and tries to use it, they have access to several features. The list is not very long, and your private data remains permanently private. No one can see your pictures, contacts, recent calls, or messages, for example. That will never change.
On the other hand, by default, with your phone locked, someone can access your control center, or at least partially see your notifications. The Control Center, in my opinion, is more relevant. From there, someone can put your phone on the plane and you can no longer receive new calls or messages. If you cut off your cell phone data and Wi-Fi connection, your phone can no longer be located. The fact that you can adjust the screen brightness or volume is the smallest issue.
Fortunately, Apple also allows you to restrict access to the Control Center or Notification Center behind the authentication process. Thus, only after you login with Face ID or Touch ID can you see that information, and no one else will be able to see / edit it, just because they got their hands on your iPhone for a few moments.
To disable unobstructed access to the Control Center and / or Notification Center, unlock your phone and enter Settings. From here, you are interested in the Touch ID and Access Code or Face ID and Access Code section. You will need to enter the phone unlock code.
On this page, the section you are interested in is "Access to lock screen", as you can see in the screenshot below. From there, turn off Notification Center and / or Control Center. The changes will take effect in real time.