The single most useful thing to know about the ankh is that it is a piece of writing. In the Egyptian script it is the hieroglyph transliterated ankh and it means life. It is used in ordinary sentences, in names such as Tutankhamun, and in formulae wishing life to the king. It is not a badge that stands for a doctrine; it is a word that was also, conveniently, a good shape.
That grammatical status governs how it appears in Egyptian art. The most common scene is a deity holding an ankh by the loop and extending it to the nose of a king or a favoured dead person. This is not the transfer of a magical object. It is a picture of a sentence: the god gives life. Ranks of ankhs with arms hold up other signs; ankhs alternate with the djed pillar for stability and the was sceptre for dominion in decorative bands that read as a wish for a long, stable and powerful reign.
It appears from the Early Dynastic and Old Kingdom period onward and remains in continuous use for some three thousand years, which makes it one of the longest-lived signs in the human record.
Egyptian mirrors were sometimes made in its shape, and mirror cases too, which is a pun on the word rather than a statement that mirrors mean life. Egyptian scribes enjoyed that kind of thing. Reading Egyptian symbolism without reading Egyptian grammar produces a great deal of confident nonsense, and the ankh is the sign it happens to most.