Gaining root access on Android

Greetings, please I have a few questions I desire to ask. Please help me.

  1. During Android penetration testing, what ways and what tools can be used to gain root access or admin privileges and to what versions is this limited to. Again where rooting is required, how can this be performed remotely?
  2. How can Android apps be installed/uninstalled silently and also made to run in stealth mode.