Why do British people call them British?

Why do British people call them British? I think it's not that I don't like this question, but I don't know who the British are referring to. All over the world, we in China call them Englishmen, and they call themselves Englishmen or Englishmen, which is omitted when we call it convenient. Needless to say, we call it Britain. The full name is: United Kingdom of Great Britain and Northern Ireland. If they meet for the first time, they really don't know when they will become' foreigners'. The English are descendants of the Anglo-Saxons, while other Scots, Irish and Welsh are natives and belong to the Celts. In addition, one believes in Orthodox Christianity and the other believes in Protestantism, so there are some contradictions, but they are not big, mainly with the Irish, followed by the Scots.