Crossplay is purely input-based so when you opt in, you and whoever is in your party will only face like-for-like inputs. If you're using a pad either on PS4 or Xbox, you will only face those players OR a PC player who is using a controller. If pad players don't mind facing KBM users, they can further opt in to unrestricted crossplay meaning they face anyone with any control scheme. Console KBM players have to play against PC users or fellow players with this scheme on console.
Crossplay has to be enabled for cross-platform play too. If someone doesn't have this enabled, they will not be able to join a party on another platform. I don't think you can sneak any crossplay changes whilst in a lobby or party as that can only be done in the menu so loopholes like this are covered. Console pad users who may want to play with a PC user with a KBM may have to have unrestricted crossplay enabled to do this. A KBM user on PC can join a (pad) console party host but they will only be matched with unrestricted crossplay users.
I'm probably gonna get it on PC which means I can use KBM against PC users and also join console users when I plug my controller in instead. When I do this, I will have restricted crossplay on meaning I will only be able to join Xbox Live, PSN and PC users who also have a bad.