Need Help Understanding Background Scaling for Mobile Devices

I'm extremely new to Unity but have been learning fairly quickly and am attempting to make my first game while I learn as I go. One of the biggest problems I have, actually it is the biggest problem I have, is understanding the graphic scaling. I've read a lot about it and watched videos on it and all of that but it still isn't clicking in my head. I've been doing graphic design for about 13 years now using mainly Photoshop and am set in my understanding of sizing and proportions and all of that. So, I'll explain exactly what I'm trying to do and hopefully someone can either explain how to do it or link me to an explanation.

I'm creating a game where the background needs to fill up the entire screen, for all mobile devices, but also fit perfectly, like not go passed the screen edges. For example, games like "Ballz" by Ketchapp (not trying to advertise for them, just first one that came to my head). Ballz background and the balls in the game fits the screen perfectly and doesn't go off screen or shrink to not fully fit the screen. Basically, what I will have is an isometric kind of perspective game and will have perfectly sized squares filling the screen, say maybe 4 up and down while having 9 side to side. (landscape mode) So please, pleaseee, help me. I can't figure this out and I'll be forever grateful to anyone who can help as I'm about to start working on the backgrounds as I type this. Thank you soooo much for any help I get!!!

 

This is a bump but also clarifying what I need. I need to understand what the ideal ratio for the background of a game is for android and ios. I know phones have different screen resolutions so what is the ratio where the background will fit perfectly on all phones? I would love if my background locked in so each side and corner was touching the edge of the screen.