How do bonds work when being traded?

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I understand that they are debt being issued with interest for a certain amount of time, but I dont get how people can sell and buy one intraday. I thought it was a fixed return and no money was made until the annual/ semi annual payment for the interest is due. In a book i read a trader would buy and sell municipal bonds intraday. So if he sold these bonds, who bought them? Is it just like any other market, I thought only companies, state, or gov could issue them and not be bought from another seller in the market./ I thought bonds were more official contracts that made you hold them until the contract is over.

Even if someone bought from a seller a bond in october, would he receive only half of the return for the year?

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Are you not reading my true question or the heading? Im assuming just the heading.

How do bonds work when being traded?