Most parents of children after divorce or separation, if not married, will resolve their custody issues with "joint custody". More often than not parents will continue to have joint custody of their children as the years go by, however that is not the reality for some families.
The words "joint custody" do not necessarily mean "shared parenting" or "equal parenting" which generally is a 50-50 or close to equal sharing of time with their children between two households.
So for parents who do not have a 50-50ish parenting plan the concept of joint custody is primarily about joint consultation and involvement in major decision making of medical, educational, religious and other significant decisions that go well beyond day to day type decisions.
The first, and likely most important, goal in maintaining joint custody is to have a civil, courteous and productive relationship with the other parent so that the children can thrive with the involvement of both parents. Parents that are cursing each other, bad mouthing the other parent to the children, using the children to convey messages, hiding important information from the other parent about the children are on the rocky road of possibly having joint custody modified to "sole custody". The label change does not necessarily mean less parenting time but it will mean less involvement and opportunity to participate in decision making on major issues affecting the children. You don't have to be "friends" with the ex, but you have to be able to get along when it comes to your children.
Parents whose post divorce/separation behavior has led to Orders of Protection, supervised visitation, or choosing new partners with addiction issues or significant mental health problems may be at risk for losing "joint custody".
If one of the two parents is consistently not utilizing parenting time (without good cause), not showing interest in the child's activities, not involved with medical decisions and/or medical treatment for more serious problems or in denial about the child's educational progress and perhaps special needs there is a risk that if the uninvolved parent is thwarting decisions involving the child's general welfare, the court if asked to intervene might terminate the "joint custody" relationship.
Staying involved and participating in decision making in a civil, respectful and productive manner is the best way to keeping your right to be consulted, having your views considered, having suggestions accepted all in the best interests of your children. Parents whose negativity toward the other parent may also lose primary residential custody because that type of behavior is often deemed by the court to be so detrimental that it will have greater weight than the involvement of the grenade throwing parent.
I once had a client who in her cellphone contact list gave the other parent the label "asshole". Every text message and email showed that was what the mother called the father. Every time the kids could see the label when her phone rang or pinged from a text from the father. Needless to say the judge was not impressed and concerned about the message the mother was conveying to the children.
In conclusion, you don't have to like each other, you don't have to be friends, you don't have to celebrate birthdays and holidays together but you must try to get along and be respectful. Of course, if you are met with the opposite behavior it will be challenging to keep your head up and stay out of the "fight". Just keep calm and remain focused on your children.