I read on PC, how can I save power and make PC last longer by under powering the PC ?

On Windows I put max CPU state to 1%, PCIe and WIreless Power to power saving mode.

Also related question, How to disallow software from inhibiting power management?

it gets very technical very quickly on Linux, but have a read here:
https://linrunner.de/tlp/introduction.html

You can use cpupower to enter different states of power limits. The link is to the arch wiki but I'd bet it's in your package manager for you to install.

https://wiki.archlinux.org/title/CPU_frequency_scaling

Not sure about the second question.

what's "linux"? which DE?

in general, using the "powersave" governer, available in Plasma and Gnome. the former can be activated automatically, the latter manually or with an extension. one of the largest consumers is the screen brightness, so you should focus on that.

also, undervolting helps if you have compatible and capable hardware.

TLP.