Marnie: Kanto. Since team Rocket disbanded you traveled to Hoenn, Unova and eventually, Galar.
May: Johto. You moved to Hoenn to be closer to your father seeing that he is a gym leader there.
Gloria: Kalos, your abilities as a ghost-type trainer got you an invitation to meet the gym leader in Galar and after a match, he offered you this interesting position as a substitute. This is also the reason why you have pokemon that wouldn't normally be in Galar
Serena: Sinnoh, Professor Sycamore met you while he was studying under Rowan, so when you moved to Kalos, he contacted you right away.
Courtney: You are native to Alola but moved quite a bit thanks to the International Police asking you to investigate different things. This time, it was the teams' activities in Hoenn.
Sabrina: Johto, you moved to Kanto shortly after being able to. Not a big reason for it, you just wanted a bit more independence.
Cynthia: Sinnoh, born there and fascinated by the discoveries made there. Your father would usually take the family on vacation and you took the chance to learn from every region you could.
Nessa: Galar, started with tough luck but with your older sister Oleana by your side, you managed to pull through.
Bea: Unova. You moved to Galar after seeing just how spacious it was, especially compared to the scenery you were used to.