How do varnas differ from castes?

Usually these concepts are perceived as some kind of synonyms in the context of Indian culture and religion, but this is far from the case. In fact, varnas are, first of all, a broader concept and mean a natural predestination to certain occupations, that is, in fact, this is the so-called “choice of the soul”. At the same time, castes are a narrower concept and the castes themselves are thousands, while castes are only 4. Castes predetermine a specific type of human activity, in which he, as a rule, remains until the end of his life.