Maybe he inherits it. In the wizarding world, there could be some old family connections or a will that passes the ownership of the bank to him.
There might be some sort of magical bargain or prophecy involved. For example, a long - forgotten prophecy stated that Harry Potter would one day own the bank to bring about a great change in the wizarding financial system, and through a series of events, this prophecy is fulfilled.
He might introduce more fair lending practices. In the wizarding world, some families or individuals might have been unfairly treated when it comes to borrowing money from the bank. Harry could change that.
He might have discovered the island during one of his adventures and claimed it as his own. Since there was no one else on the island at the time, and he was the first to set foot on it and recognize its potential, he decided to make it his property.
It could be that the dragon was an egg he found by accident. He, being curious and having a soft spot for magical creatures, took the egg and hatched it, thus becoming the owner of the dragon. In the wizarding world, there are often unclaimed or lost magical items and creatures, and Harry might have just stumbled upon this dragon egg.
There's also a possibility that it's a Chinese Fireball. These dragons are very beautiful and powerful. In a fanfiction, the author might choose this type of dragon for Harry Potter to own because it would add an element of exoticism and grandeur to the story.
Perhaps he discovers the planet during one of his adventures. He could be using a new spell that accidentally transports him to this uncharted planet, and then through some series of events, he becomes the rightful owner. For example, he might save the planet from a great evil and as a reward, the planet's native beings make him the owner.
Well, it could be a magic item shop. Since Harry Potter is from the wizarding world, he might sell things like wands, potions, or magical artifacts in Konoha.
There's a possibility that after Sirius's death, Harry just sort of took over. He had already spent time there and knew the place well. The remaining members of the Order of the Phoenix, seeing his connection to the place and his leadership potential, supported his claim to Grimmauld Place.
He probably uses magic. For example, to water the plants, he can use a spell instead of a regular hose. And to fence the ranch, he might use a charm to create a magical barrier that can keep unwanted animals or people out.
He might use his magic skills to stand out. For example, his ability to perform spells could be seen as a unique talent in Konoha. His shop could become a place where ninjas are curious to visit and learn about different kinds of magic.
It adds a new element of defense. Traditionally, Harry uses his wand and spells for protection. But with a shield, it gives him a more physical form of defense. It can change the way battles are fought in the story.