Dukedoms are normally hereditary, but the sovereign can grant honours on any terms they like. The king has just made his brother Edward Duke of Edinburgh on a non-hereditary basis.
There are certain ducal titles that are commonly awarded to members of the royal family - e.g. the sovereign’s second son is traditionally made Duke of York. But in the nature of things its always possible that they will descend to non-royals. If a royal duke has sons, and they have sons, and they have sons, and so on, after two generations the title is held by a non-royal — this is likely to happen with the Dukedoms of Gloucester and Kent. Eventually the title may fail for want of heirs, in which case it becomes available to be granted to a royal again, but it is not inevitable that this will happen.
One of the odd things about the title Duke of York is that ever since it has been customarily granted to the second son, it has never descended to an heir. The second son of the monarch has either died without male heirs (as will likely happen with Prince Andrew), so it reverts, or the second son has become king, as happened with George V and George VI.
It’s traditional custom and practice to give male children of the monarch secondary titles acknowledging each of the nations/provinces of the UK. So as the eldest William isn’t just Prince of Wales and Duke of Cornwall, but also automatically Duke of Rothesay, Earl of Carrick, Baron Renfrew, and for good measure Lord of the Isles (tenure of which mattered to the mediaeval kings of Scotland). Likewise Harry is not only Duke of Sussex, but was also made Earl of Dumbarton and Baron Kilkeel. The same was done for the brothers of George VI (Dukes of Kent and Gloucester) and their descendants.
The assorted titles for Victoria’s younger sons died out, AFAIK mostly for lack of successive male heirs.
Or because they were a bit too German. The titles of Duke of Edinburgh and Duke of Connaught died out in 1900 and 1942 respectively for want of heirs, but the Duke of Albany was deprived of his title in 1919 for having togged out for the wrong team in the Recent Unpleasantness. There is an heir; in theory Hubertus Prinz von Sachsen-Coburg und Gotha could petition for the restoration of the title.