The European Union (EU) isT a political body in Europe that is composed of 28 European states. This number will soon (end of October, after the topic is finished) be reduced to 27, as the UK is leaving the EU.
The EU was formed after World War II with the goal of building stronger economic and political ties among countries in Europe with the goal of reducing conflict, as two wars were fought on the continent.
Encyclopedia Brittanica explains:
European Union (EU), international organization comprising 28 European countries and governing common economic, social, and security policies. Originally confined to western Europe, the EU undertook a robust expansion into central and eastern Europe in the early 21st century. The EU’s members are Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, and the United Kingdom. The EU was created by the Maastricht Treaty, which entered into force on November 1, 1993. The treaty was designed to enhance European political and economic integration by creating a single currency (the euro), a unified foreign and security policy, and common citizenship rights and by advancing cooperation in the areas of immigration, asylum, and judicial affairs.
The EU acts by trying to form common economic, environmental and security policies.
The European Parliament is the EU’s law making body and its membership is directly elected by the citizens of the EU member states. The Council of the European Union represents the governments of the EU. The European Commission represents the interests of the EU as a whole.
The European Commission is the executive arm and is responsible for drawing up new legislation. It implements the decisions of the Parliament and the Council.
If the EU were to join the BRI, such a deal would be most likely be negotiated by the European Parliament for the purpose of reaching a Common Commercial policy
The European Union‘s (EU) Common Commercial Policy or EU Trade Policy is the policy whereby EU member states delegate authority to the European Commission to negotiate their external trade relations, with the aim of increasing trade amongst themselves and their bargaining power vis-à-vis the rest of the world. The Common Commercial Policy is logically necessitated by the existence of the Customs Union, which in turn is also the foundation upon which the Single Market and Monetary Union were later established.
It also seems reasonable to conclude that all states were obligated to the decision.