"Customary" is derived from the word "custom". Customs, sometimes synonymous with traditions, are a construct of humans. You would use the word "customary" (or "customarily") in connection with a pattern of behaviour established by a person, or people. As it is quite specific to say something is a person's custom, you would use this about a specific person or group of people. You can't really generalise about the customs of people you don't know or do not identify in your statement.
What is "usual" can be determined by many things beyond human control. For example, it is "usually" cold in winter. You can use this word in connection with just about anything that has a consistent, but not rigid, pattern. You can also use it in a broader sense than "customary", such as in a generalisation about people you don't know, for example, "tourists usually visit the Eiffel Tower when they go to Paris").