As a third and best reference you should watch Mr. Robot, the TV series. There is a scene, where a victim opens a picture on his Apple device, allowing an attacker to gain complete access to the device. Mr. Robot is one of the most accurate screenplays in regards to hacks, exploits and computer business.
No, anti-virus software is overrated.
The best thing you can do is to only use trusted sources. For example, only use FOSS. Only download from people and entities you trust.
Of course, nothing can make the computer a 100% safe, in the end.
There is only one way to be absolutely safe:
Remove all network hardware from your computer and never put any 3rd party software on it.
Ideally, you should manufacture the hardware itself, too.
This is the only way to be absolutely sure. And even then, you must make sure not to let any government agents near the location where the hardware is placed…
