The hieroglyphs correspond to sounds or letters. The lion, the 2nd letter, for example, is an L.
Champollion discovered it because names were written with the hieroglyphs.
Reading Champollion’s letter to M. Dacier written in 1822, he explains exactly how it is done, addressing the names Cleopatra en Ptolomée.
Will elaborate more. In synchronicity there is currently an exhibition in the British Museum offering a detailed history of this https://www.britishmuseum.org/exhibitions/hieroglyphs-unlocking-ancient-egypt/egyptian-hieroglyphs-decipherment-timeline