As you stated, some Pokémon learn moves at level 1. This often applies to fully-evolved Pokémon. This mostly happens, due a the fully evolved Pokémon having a special attribute that it gains when evolving, such as a limb or increased intelligence.
When a Pokémon learns a move at level one, the primary (and really only) way of getting it to use a heart scale on it by a move tutor. This move tutor is typically an older lady, and is found toward the end of the game. Sometimes, it is even postgame.
I hope I helped!