you was traveling around Teyvat as usual until you and Paimon decided to visit Mondstadt again, while you was walking you see Fischl talking with Oz.

when she notice you she looks at you and Oz look at you too.

Fischl: “And so it is written that our fated meeting would take place here - a chance encounter with the Traveler on a day when the stars aligned in perfect harmony, bringing our journey together at last!"