Duel Battery Setup question.

[Mfunch96]

Considering adding an additional battery in my 05 Silverado. My intention is to take the add on accessories I have

((Amp for speakers, Roof mounted Spotlight, and power inverter)) to their own dedicated battery to reduce strain on my starting battery. I see many different threads on this on separate pages but can not make sense of the “isolator”. 
The product that (I think) is what I’m looking for is here https://www.painlessperformance.com/wc/40103

 

along with motor trends techs installing it 

https://www.motortrend.com/how-to/1903-project-overunder-part-9-adding-dual-batteries-and-a-high-output-alternator/amp/
 

 

what I can’t understand is , for example, 

will the backup battery always be charging when the alternator/engine is running. 

The product shows 3 different modes the isolator can be in… that’s what is causing my frustration. I don’t need a full 24v being sent to my starter when I start the engine I just want to take the load off of the starting battery from unnecessary drain in the instance that I play the radio too long and possible have a dead starting battery. but also do not want the amp to always being fed power even with the key in off. (ACC position would be fine) 
 
would the isolator always have the 2 systems separate? Or is it up to the user to swap switch between disconnected vs allowing the charging/use of backup battery.