No it's not as well Duchy of Lancaster and Duchy of Cornwall the government takes most of it and just gives them a tiny sliver
The duchies are not the private property of the King Charles and Prince William , or the “crown”. They are properties owned by the government whose rents are used as income by the Sovereign and the Heir Apparent, since they don't hold jobs and earn income like regular people.
should the monarchy end and the UK become a republic, that the Windsors will keep the lands. Not true. Not true. Not true. They have access, but do not own. Just like the Crown Jewels, the palaces, the royal collections, etc...
The only thing they really own is Balmoral, Sandringham , stamps and some art collection