Here "a heart" refers to their desires. Alternate translation: "I will cause them to desire to know me" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here "all their heart is an idiom that means "completely." Alternate translation: "they will turn to me completely" or "they will turn to me without reservation" (See: [[rc://en/ta/man/translate/figs-idiom]])