How do TV/ Radio networks earn?

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Have you ever wondered how do these TV networks earn? I first heard that it's from advertisement. Companies who provide services and products advertise their business to the public who are potential consumers. These companies pay the the network companies for the medium through which their advertisement are delivered to the people - those are, via the TV and radio. There may be many other ways. But let's focus on the two - TV and radio since they are two of the oldest, through use of technology. Wondering, is there any other way that TV/radio networks earn besides from payment the advertisers pay them? How else are they funded? Or is it the only way they earn? They deliver programs to attract people to watching, or tuning in to them. Those programs may be educational, entertainment, or just about of any interest, to reach the most, and that's maybe part of the reason they become the media. Did it happen because of the competition between companies. So many products, they want theirs to be noticed, so they will pay for that media to get it broadcasted so will know their products exist, its features, and how it is better than the other. Without these companies advertising their products will there be TV and radio stations for free?