A police officer arrests you and puts you into the back of the squad car. You have no idea what you did wrong. Maybe you do not think that you broke any laws. You demand to know why they're arresting you. Do they have to tell you?
While they may choose to do so, law enforcement officers typically do not have to inform you why you're being arrested. They are not always required to tell you what's happening or why, especially if they do not think that it makes sense to do so at the time. Arrests are often chaotic.
Generally, arrests are made on the basis of probable cause or a warrant, just like home searches and other police actions. If police get an arrest warrant, it means that they believed they had reason to arrest you and took the time to go to the court first. A judge issued the warrant. They may tell you that they have it, they're not required by law to show it to you before taking you in.
An arrest based on probable cause may happen in the moment. An officer may think that they saw you commit a crime. There is no time to seek a warrant. The officer will just arrest you and later explain their reasoning to the court. Again, though, they may not tell you what they saw or what they think you did wrong.
The key is to remember that police must have a reason to arrest you. You will learn what it is when you go through the legal process even if you do not learn why during the arrest itself. Make sure that you seek legal help before facing the justice system.