The asset is well polished for a *Starter asset* . And as mentioned in the video, all of it is a pretty good base, the asset is made as a base for you and other awesome creators to build their own custom controller upon and use, although it's a great well rounded and polished character controller it's still a base and just a starter asset from the name.

I think if it is a pre-made animation that you imported from somewhere then you might not be able to modify it, but if you made that animation in unity then yes you can modify it any time, and usually the animation is rig based and you don't just move the mesh without bones or the rig

But also you can import different animation that suits your character instead of the default animation and just replace them in unity.

a 2D blend tree works by having a velocity parameter calculated in code then applied to the blend tree value, there are a lot of videos on KZfaq explaining it really well, so hope you can find what suits you!

if you just installed the package and open the provided scene you shouldn't have any problem, so maybe you edited something by mistake?

The asset was made for the built-in renderer in mind, so the materials will not render in URP unless you convert it, hence for why unity made that handy tool that lets you convert built-in materials to URP materials. It is easy to change from one to another but it's just in different places since there is a lot of tools in unity haha.