That's actually what I originally wondered about lol. Whether Wales really is not part of England
But like, is "England and Wales" like legally a thing
Yes; there is a common legal jurisdiction for England and Wales.
However the Welsh have their devolved national assembly for Wales.
Welsh is spoken in Wales, but it is only really predominant in the North of Wales.
There are cultural and even some genetic distinctions between the Nortth and South Wales.
or just "The part of Great Britain that isn't Scotland or Northern Ireland"?
Not quite.
Northern Ireland is part of the UK, but Northern Ireland is not part of Great Britain.
Great Britain is in essence one large Island, although it is customary to refer to small adjacent
islands such as the Isle of Wight, Lindesfarne Island and the Scilly Isles as being part of Great Britain.
Other islands such as the Channel Islands and the Isle of Man are neither part of GB nor the UK
although the UK represents thenm for the purposes of international things such as EU, NATO or UN.
It was named Great Britain to distinguish it from Brittany which is in fact the north west tip of France.
Remember, it is the United Kingdom of Great Britain and Northern Ireland.
The legal system of Northern Ireland is very much like the legal system of England
and Wales, although there are some differences in laws.
The legal system of Scotland is significantly different from that of England and Wales.
They currently have three sets of applicable laws in Scotland:
(i) European Laws,
(ii) UK Laws; and
(iii) Scottish Laws.