Correct use of a child theme

Hello,

I'm posting to clear something up that I can not really definitively find in other support topics.

I have GeneratePress Premium + GenerateBlocks.
I have taken a site template from the GenPress site library and begun to modify it to my own design, using it as a template and a basis for learning about GenPress and GenBlocks.

I have just read about child themes.
The template came with CSS in the Additional CSS section of the customiser. I obviously want to protect that CSS from being overwritten. This is where my child theme questions come in. As I understand it when GenPress gets updated the CSS will be overwritten - is that correct? Or am I confusing that process with something else - i.e modifications to other CSS files somewhere else in the theme? on updating GenPress does my additional CSS stay intact?

Basically - going forward should I use a child theme to add and protect my CSS or continue to add any CSS via the additional CSS part of the customiser. Am I worrying about something that I have misunderstood? I don't want to continue working on the site if the CSS could be overwritten.

I have seen that a simple way to add CSS is to use the Simple CSS plug in. I can see that it has not been updated in a year. Is it going to continue to receive updates and support?

Many thanks in advance

Matthew