Groups created on O365 are added as a user with a mail system as O365. So mail sent by any SkyConnect user will be delivered to the group id on O365 from where it will be distributed to all the members.
Groups on SkyConnect are created as contacts on O365, so a mail sent by an O365 user will be first routed to the group id on SkyConnect from where it will be distributed to all the group members.
Recommendation: While groups can contain mixed email ids from across both systems, it is advised that if a group contains a bulk of ids from O365, then the group be made on O365 and vice versa to reduce mail traffic between the two systems and optimize local delivery.