A reservation is recognized as a "returning guest" if the guest's name and contact details, i.e. phone number and email address, match a previous reservation in Guesty. The new reservation will be marked by a star that reads "Returning guest" in the Guest section of the reservation page, under "Name".
The credit card used for a returning guest's new reservation is the one saved for that guest. If you have multiple Stripe accounts, the account in which their credit card was originally stored will be charged. For example, if a guest stayed at listing A that was assigned to Stripe account A and listing A is moved to Stripe account B, account A will still be charged for that returning guest's new reservation.