For years, NextApp's SystemPanel has been a popular way to quickly monitor almost all aspects of an Android device. The app supports monitoring running applications, hardware information, network activity, and more. But like many pro tools for Android (looking at you, Titanium Backup), the interface is a bit dated.
Left: SystemPanel 1.4, Right: SystemPanel 2 Beta
SystemPanel 2 is currently in public beta, and just like the earlier version, shows a wealth of information about your device. In the new update, not only is the interface updated to Material Design standards, but everything in general is much easier to access. If you're on Android 7.0 or higher, some features (such as determining background applications) will not be available due to security limitations, unless you are rooted.
The original version is available in free and pro versions, with the latter removing all ads and costing $2.99. With SystemPanel 2, you can buy SystemPanel Plus (which also removes ads) as an in-app purchase for just $1.99.