Answer : Basically react create a virtual dom as like as real dom. When change any thing in virtual dom, then react change the real dom using a efficient algorithm. Basically react application is the set of components. Here use more components. Components means part of the body. A navbar is a component of a website. React component return JSX , which mean Javascript Object Notation. You can write here HTML code, but not exact HTML code. It's give you just syntactical sugar.

Answer
Props: Basically props work for data passing from parent to child component. It's immutable, can't be modified. Props are read only.
State: Here data pass only in the own component. It can be modified. In this case read and write both can do.

Answer : useState is a react hook, where has a variable and setter function, If we can change the varible value, then we need to change it using setter function.
For Example : const [state, setState] = useState(initialstate). Here state is the variable or a state and setState is the setter function , it change the state value.