England, Scotland, Wales and Northern Ireland are separate countries or nations. They’re not nation-states. For all practical purposes other than sport*, they are administrative divisions within the nation-state called the United Kingdom. They are effectively the same as US states, except that they don’t have sovereignty; Parliament can theoretically dissolve their legislatures whenever it likes and administer them directly. That, and England doesn’t have its own legislature; it’s like DC.
*sports makes everything a million times more complicated. In association football, each country has its own team, but in rugby each country has its own team except Northern Ireland, which fields a joint team with the Republic of Ireland.
In track and field (“athletics”, in the UK), Northern Irish competitors used to compete for “Ireland”, while English, Scottish and Welsh ones competed for “Great Britain”. Now Northern Irish competitors compete for “Great Britain and Northern Ireland”, except the ones who don’t, who compete for Ireland.
In motorsports, drivers compete for “Great Britain”, including the ones from Northern Ireland, which is not part of Great Britain. Don’t ask me how that works.
Oh, and then there are the Lions, who are a combined rugby team made up of players from England, Scotland, Wales, and the Irelands.