It does not replace my MBAir. There are things that are easier to do on an iPad and vice-Vera. The iPads is lighter.

I had to buy a new pencil because the old one went to my wife with my old iPad. I mostly use the pencil for marking up pictures with arrows or text, but sometimes play in ProCreater.

Yes, major differences. MacOS and iPadOS are very different. I actually like iPadOS more, but there are things it cannot do — and vice-versa, as I said above.

Some apps work exactly the same, some are slightly different, some are vastly different, some only exist in one place.

Two big things for me:

You cannot run Xcode on an iPad.

You cannot use your finger or a pencil on a Mac screen.