Can a “bad guy” spy on an android phone without installing spy apps (those apps that require payment)? By “spying”, I mean using key logger, screenshots, read messages and so on.

Is it possible to spy on an android without using such spy apps? Is there another way someone can spy on you?

Does “unknown sources” have to be turned on for spyware to be installed?

Can someone install some type of spying software or are spying apps the only way?

Let's say the “bad guy” has physical access to your Android device, how much time would that person need in order to install some type of spyware?

Basically, how do people install and spy on android phones?

I am worried that this might have happened to me, so if someone can explain this to me, I will be very thankful! Also, if my questions sound silly please keep in mind that I am not educated enough about this topic that’s why I am here.