Broadcasting equipments are usually very expensive and doesn't make business sense to purchase them for a particular broadcast right wherein they are to be used only for a few days in an year. This is where companies like Sunset+Vine, IMG etc. step in. They solely deal in broadcast production all around the world. They may sometimes directly get the rights or may be sublet by TV Channels like Star Sports, Sky Sports etc.who hold the production rights.
Now being a Civil Engineer, I'll try to explain by an example. We usually see big construction firms like L&T win government contracts worth thousands of crores. Now, a construction project involves 'n' number of big and small activities such as Piling, Concreting, Shuttering, Plumbing etc. L&T doesn't necessarily hold expertise or resources for every activity, so they sublet these activities to smaller companies who hold expertise for a particular activity. So, there a number of sub-contracts within a single contract. These sub-contracts are dealt by L&T directly.
On a similar basis, Star Sports holding the production rights for IPL may actually sublet it to a specialist company like Sunset+Vine for broadcast production. It may be easier for them to customise the feed as per their own requirement.
Sometimes, these production companies get direct rights from the authority. For example, Sunset+Vine has production rights of ICC, IMG has production rights of ATP Tour Tennis events and so on.