This depends on your definition of informal terms like "letter", "character" etc.
The typographic term for combinations like this is "digraph". (Wikipedia's definition: "A digraph [...] is a pair of characters used in the orthography of a language to write either a single phoneme [...] or a sequence of phonemes that does not correspond to the normal values of the two characters combined".)
Whether digraphs have separate keys on a keyboard, are treated as distinct for the purposes of alphabetisation, whether speakers of the language think of them as separate "letters" when spelling out a word and so on, are all separate issues and varies between languages (or, more precisely, between the conventions for writing a certain language).
The typographic term for combinations like this is "digraph". (Wikipedia's definition: "A digraph [...] is a pair of characters used in the orthography of a language to write either a single phoneme [...] or a sequence of phonemes that does not correspond to the normal values of the two characters combined".)
Whether digraphs have separate keys on a keyboard, are treated as distinct for the purposes of alphabetisation, whether speakers of the language think of them as separate "letters" when spelling out a word and so on, are all separate issues and varies between languages (or, more precisely, between the conventions for writing a certain language).