The sangha has no head. That is the first thing to understand about it and it is the thing most Western descriptions get wrong, usually by looking for an equivalent to a pope, a synod or a governing body and then reporting whichever national office most resembles one.
In its primary technical sense the sangha is the community of fully ordained monks and nuns. What holds it together is not an organisation but a rule and a procedure. The Vinaya sets out the training rules and the formal acts a community must perform, and the central one is the fortnightly assembly at which all monks within a defined boundary must gather to recite the rule and declare their breaches. Attendance is not optional and the boundary is a real spatial thing that has to be formally established.
Decisions are taken by the monks present within a boundary. That makes the sangha a federation of local assemblies rather than an organisation with levels. There is no body that can define doctrine for Buddhists, no procedure by which the Thai monastic community binds the Sri Lankan one, and no office from which anyone speaks for Buddhism.
Two further points guard against predictable errors.
First, the national monastic hierarchies that do exist, with supreme patriarchs, government-appointed supervisory councils and formal ranks, in Thailand, Cambodia, Myanmar and elsewhere, are creations of states from the nineteenth century onward. They are not developments out of the Vinaya and they should not be read as the sangha's own structure. Where a state built such a body it has generally also used it, and the entanglement of monastic authority with government is a live and sometimes ugly issue in several countries.
Second, sangha in broader usage, especially in Mahayana contexts and in Western convert communities, means the whole body of practitioners including laypeople. Both senses are legitimate and they are not the same sense, and a sentence that slides between them without noticing is describing nothing in particular.