Spanish is the 2nd most spoken native language worldwide. There are appr. more 520 millions of them. (English: 360 millions native speakers)
Spaniards sometimes speak Castilian to illustrate the difference with other languages used in Spain such as Galician, Catalan, etc.
According the 2018 figures of German Wikipedia from 2018, Catalan is spoken by about 9.2 million people. This number includes those who were forced to speak the language. The number of native speakers for the same year in English Wikipedia is given as 4,1 Mio.
Catalan is spoken in Spain mainly in Catalonia and the Balearic Islands. It is the language of a strong minority, mainly in rural areas. The majority is Spanish speaking.
Valenciano is a dialect of Catalan and, according to German Wikipedia, is spoken by about 2.4 million in 2018. English Wikipedia counts 2.4 Mio as native speakers, thus the number of people, on whom Valencia was imposed in school, is not clear. However, the exact numbers in this case seem not so important.
Valenciano is mainly spoken in the Comunidad Valenciana. Here, too, this language is used by a large minority, especially in rural areas; the majority is Spanish-speaking.
How can you tell the difference between Spanish and Catalan/Valenciano?
The most Spanish nouns end with "a" or "o" in singular or in plural with "as" or "os".
Words, ending in English with "tion" or "tions", like i.e. Nation/Nations, Construction/Constructions, Action/Actions etc., end in Spanish with "ción" or "ciones". Example: Construcción/Construcciones.
Catalan nouns rarely end with "a", "o", "as" or "os".
"tion" or "tions" is in Catalan "ció" or "cions". Example: "Construcció/Construccions"
If you read only "ajuntament" at your town hall and don't find the Spanish word "ayuntamiento" anywhere, then you know that they are already trying to suppress Spanish, even though according to the corresponding constitutions of the autonomous communities it is one of the official languages.
Official languages in Catalonia are Spanish, Catalan and Aran. In Valencia they are Spanish and Valenciano.