We first discovered that in Barcelona, Spain, where Catalan is spoken. Barcelona is the capital of Catalunya and both Spanish and Catalan are spoken and often interchangeably. Basque, Galician, and Extremaduran are other "Spanish" languages found in areas of the country. Then you have Andalusian in southern Spain and Murcian in the southeast. Oh, by the way, Castilian is the official language of the country and is what is usually taught in schools and colleges.
As you branch out to the Spanish speaking Canary Islands, you find the dialect called Canarian. On Gibraltor which is a British territory you will hear Llanito, a combination of Andalusian and British English.
Crossing the sea to the Americas, you discover Latin American Spanish spoken in Mexico, Columbia, Peru, and other Central and South American countries. Even so, there are more dialects to be found. For instance, there is Rioplatense, a dialect in areas of Argentina and Uruguay and Caribbean Spanish found mostly in the islands of the Caribbean.
Perhaps my favorite Spanish dialect discovery is found in Africa. Equatoguinean is a combination of Spanish, the language of New Guinea and some German dialect.
All of this just goes to show that people adjust as they come together and find the need to communicate. Languages blend to meet the needs of those who live together. Communication is central to understanding and thriving in any nation.