Why Battery Care Mode if bypassing charging? How to charge slower?

I was trying to solve the question of the 3rd paragraph and found How to extend the battery life of your ROG Ally. Although the article contains a desktop screenshot, it's not explicit there's Windows 11 on it, and since this site insist on the version it's 22H2. The article mentions the device supports bypassing charging, which I think is great as I lost a battery from inflation while I kept it plugged for long periods and slowly got bigger, which I'm assuming the battery kept getting down a few % then charging back up to 100%.

But then in the article it mentions to also use Battery Care Mode so the battery by default is never charged beyond 80%, which make sense if you want the device to keep charging as soon as it goes a few % below 80. Those 2 things together don't make sense to me; the 2nd is like having an automated water bowl that fills itself as soon as the bowl is near being empty, and the 1st is like there was no water tank nor water hose plugged into the automated water bowl (making it a cumbersome bowl working exactly as a plain bowl). Can someone explain how the 2 battery modes work in conjunction or in parallel? In case you didn't look at the article.

The initial question is that the ROG Ally has a fast charger and I thought regular charging make batteries last longer so I'm wondering how to turn the fast charging off when not in a hurry. About my plain batteries (eg AAA), I have 2 chargers, the 1st is ~8 hrs and the 2nd is ~90 mins, and I seldom use the 2nd in the hope the batteries last longer.

I'm not sure where to post this question but a handheld device (eg PSP) is like a keyboard-less laptop to me.

Thank you kindly for your help

I'd be inclined to spend more time using it than worrying about it.