Rotten wooden door

I’ve recently moved into a property so it’s my first effort at any kind of DIY. One issue we have is that the garage side door has rotted at the bottom, it doesn’t look like it was provided suitable protection.

What is the best way to do this? I’ve heard of epoxy fillers but will this be suitable for the extent of rot? Thought about cutting away the rot and adding new wood in.

Looks like the base of the frame has been knocked out - can still see the nails that would have held it in place. I'd repair the frame and replace the outer cladding (looks like tongue & groove) that is missing.

Don't just fill it with expanding foam like what has been done on the other side !

Thanks dannymassive, what is the best way of repair? Can I cut away and add in? I’m not sure what they expected the expanding foam to do but it’s just turning green!