What am I agreeing to when I install an app?

Installing an app is a small agreement: you let it onto your device, and often you agree to share data and grant access to parts of your phone. Most apps ask for more than they strictly need. You do not have to say yes to everything, and it is easy to check and trim.

What you are actually granting

Permissions. Access to your location, camera, microphone, contacts, and photos. A torch app does not need your contacts; a game does not need your location.
Data collection. Many apps gather usage data and identifiers, sometimes to share with advertisers.
Background activity. Some keep running or tracking when you are not using them.

How to keep it minimal

Grant permissions only when they make sense for what the app does, and choose "while using the app" over "always" for location where offered.
Prefer "ask app not to track". Both iPhone and Android let you stop apps following you across other apps - turn it on.
Review permissions after installing. Your phone's settings list what each app can reach; revoke anything that does not fit.
Stick to the official app stores, and glance at who made the app and its reviews before installing.
Delete apps you no longer use - a dormant app is still collecting.

If you only do one thing

When an app asks for a permission, pause and ask whether it actually needs it for the job you installed it for. "No" is usually a safe answer, and one you can change later.