NetGuard

Lets you use CloudFlares fast and secure 1.1.1.1 DNS, with DNS over HTTPS, and also has the option to enable CloudFlares WARP+ VPN.

Android Firewall+ (AFWall+) is an advanced iptables editor (GUI) for rooted Android devices, which provides very fine-grained control over which Android apps are allowed to access the network. Get from F-Droid

This application blocks ads and trackers, doesn't require root and works for all the apps on your Android phone. Check out how it works here.

Gives you the ability to grant permissions temporarily, so that you could for example use the camera to take a profile picture, but when you close the given app, those permissions will be revoked.

Prevent background processes gaining unauthorized access to your devices camera. Better still, use a webcam sticker.

Simple tool, that compares SHA-1 fingerprints of the the SSL certificates seen from your device, and the certificate seen from an external network. If they do not match, this may indicate a man-in-the-middle modifying requests.

Encrypts files and folders client-side, before uploading them to cloud storage (such as Google Drive, One Drive or Dropbox), meaning none of your personal documents leave your device in plain text.

Shows which trackers, each of your installed apps is using, so that you can better understand how your data is being collected. Uses data from the Exodus database of scanned APKs.

F-Droid is an installable catalogue of FOSS applications for Android. The client enables you to browse, install, and keep track of updates on your device.

A network scanner to help you monitor and secure your WiFi network. The app is totally free, but to use the advanced controls, you will need a Fing Box.

Auto-disable Bluetooth, then it is not being used. Saves battery, and prevent some security risks. Get from F-Droid

An actively-maintained fork of the Island project with additional enhancements

A sandbox environment, allowing you to clone selected apps and run them in an isolated box, preventing it from accessing your personal data, or device information.

System-wide Tor proxy, which encrypts your connection through multiple nodes. You can also use it alongside Tor Browser to access .onion sites.

Auto lock your phone based on movement force/ acceleration. Get from F-Droid See also PluckLock

An open-source ad-blocker and firewall app for Android 6+ (does not require root).

Prevent background processes taking unauthorized screenshots, which could expose sensitive data.

Triggers actions, when certain security conditions are met, such as multiple failed login attempts or monitor settings changed. It does require Tasker, and needs to be set up with ADB, device does not need to be rooted.

A radio frequency scanner app for Android with recon and offensive capabilities.

Makes it possible to entirely freeze all background activities on a per-app basis. Intended purpose is to speed up your phone, and prolong battery life, but this app is also a great utility to stop certain apps from collecting data and tracking your actions while running in the background. See on F-Droid

Monitor and control hidden data collection in mobile apps about user behavior/ tracking. Get from F-Droid