How to grant certain apps root permission through ADB? I have an Android TV box (4.4.2, armv7), and I can login the device as root by ADB (local network). What I want to do is to give certain apps root permission. How can I do it?
You don't give certain apps root permission, but certain apps ask for root permission by executing su binary. It's the developer of the app who decides if the app would perform some tasks with root privileges or not. In order to let apps request root privileges, you need to root your device using e.g. Magisk, SuperSU etc. (whatever works on your device).

When you get root privileges on adb (e.g. by executing adb root or adb shell su) it runs adbd process on device as root (UID 0) user. So all commands which you run through adb have root privileges. To grant root access to apps you need a special su binary which is not shipped with Android.

The usual su binary (like on any UNIX-like OS) makes a transition to UID 0 by making setuid syscall to kernel. This is not possible on Android due to certain restrictions. The special su binary (the one provided by rooting solutions like Magisk) connects to a background daemon to get a root shell for the requesting app, where the app can execute commands with root privileges. However the app itself is still running with its own UID (assigned at the time of installation).

For more details see:


What special privileges “/system/xbin/su” does have w.r.t. root access?
How Magisk works?
How to manually root a phone?