One app that’s worth knowing really well, is the settings app.
From the settings app you can resolve most problems that you may have with your phone, like being unable to connect to the internet or bluetooth speaker.
You can also control how secure you device and data is by modifying your passwords and restricting how much personal information an app can take from your phone.
Another worthwhile thing to know is that in the settings, you can modify how accessible your device is. This covers increasing the default text size, controlling the device with your voice, getting the device to verbally relay each action you take, making the device high contrast any much more.
To find the setting, look for icons similar to those below. On different device they can look different but nearly all device use some sort of cog to indicate the settings app.