What is the difference between in united states and in the united states?
Feel free to just provide example sentences.

"in united states" is incorrect and "in the united states" is always correct because "the" is used for specific nouns. "United states" is a noun and is a group of states, these states cannot be found anywhere else. That's why "the" is used.

Ex: The dog shat on my shoe.

Not just any dog but a specific dog.

Ex: I want a dog.

Any breed, nothing specific.

Ex: I like Japan.

Japan also cannot be found anywhere else, it's a noun but does not have a group or a noun that is specific like "the United Kingdom," so no article is necessary.

I hope that made sense. (T^T)