Watchdog timer fuse

Can someone please explain to me, or show me to a link that explains how the watchdog timer always on fuse is supposed to work?

When enabling the watchdog timer from a sketch with say "wdt_enable(WDTO_15MS);" its obviously starting the timer with a 15MS countdown. But when I enable the fuse byte, there is no specification as to how long the watchdog timer is going for. I feel like I am misunderstanding how the watchdog timer always on fuse is supposed to work.

Why do I need to enable it in the fuse settings instead of just at the beginning of my sketch?

I've been trying to debug an issue with my project where is doesn't seem to boot if I power down and up my device within 1.24 - 1.28 seconds. It is the atmega328PB on a custom board. I have successfully enabled and confirmed the "watchdog timer always on" fuse bit is set using avrdude btw.

I would simply enable the watchdog timer at the beginning of my sketch, but for some reason the boot up doesn't always make to the sketch. I know this because in my void setup, I set an LED to HIGH and it doesn't turn on. So if I could enable the watchdog timer in the fuse bits, then when it fails to start running my code, the watchdog timer would reset the device and I would simply just disable the watchdog timer at the beginning of my code.

Not an answer about "always on", but if you're using a bootloader on your Arduino the below more than likely applies.

The boot loader does not disable the watchdog timer at startup; so if you set a short time, it will keep on rebooting /restarting the boot loader.

So I beleive I got the watcdog timer working correctly, but it didnt' seem to fix the issue.
I tried using the 328P chip and the issue went away. So with the same fuse settings and the same bootloader and the same sketch, the two chips act differently. I will start a new thread to discuss this and try to get to the bottom of it