Who made him the king? Often, he did. The fellow with the biggest sword.

Why is a German the queen of England? Because, often, strategic alliances have been forged through wedlock.  As you will learn, the first of Henry VIII’s half-dozen unfortunate spouses was Catherine of Aragon.  Aragon was part of Spain.  That marriage was precipitated by Henry’s father, who wanted to cozy up to Isabella & Ferdianand.

How can someone usurp a throne? See Biggest Sword Theorem, above.

Whats a claimant? Someone who thinks that he or she should be on the throne & isn’t.  Someone in the market for a bigger sword.
This last answer begs a final question that baffles many in the American audience & is worth asking as we begin our journey: What’s with all these “houses”? The Plantagenets, the Lancasters, the Yorks, the Tudors? Who are these Windsors? The answer is that, basically, these are all families; they hand down the crown until the lineage runs dry & then a distant relative might be found in another family with a different surname, or they may simply change the surname, as the Windsors did, or a guy with a really big sword might come along & …