What is determining factor in which parent gets custody?

There is no "one determining factor."  Honestly, it is a bit subjective and ambiguous--giving wide discretion to the judge.  The judge decides which custody arrangement is in the best interest of the children.

Read Family Code Section 3011, it describes what the "best interest of the child" means.  In addition, Family Code Section 3020 states that the part of 3011 that says, "health, safety and welfare" is the most important part.  Under Family Code 3040, the judge should also give preference to the parent who is going to encourage frequent and continuing contact with the other parent.

Here is Family Code 3011:

3011.  In making a determination of the best interest of the childin a proceeding described in Section 3021, the court shall, among anyother factors it finds relevant, consider all of the following:   (a) The health, safety, and welfare of the child.   (b) Any history of abuse by one parent or any other person seekingcustody against any of the following:   (1) Any child to whom he or she is related by blood or affinity orwith whom he or she has had a caretaking relationship, no matter howtemporary.   (2) The other parent.   (3) A parent, current spouse, or cohabitant, of the parent orperson seeking custody, or a person with whom the parent or personseeking custody has a dating or engagement relationship.   As a prerequisite to the consideration of allegations of abuse,the court may require substantial independent corroboration,including, but not limited to, written reports by law enforcementagencies, child protective services or other social welfare agencies,courts, medical facilities, or other public agencies or privatenonprofit organizations providing services to victims of sexualassault or domestic violence. As used in this subdivision, "abuseagainst a child" means "child abuse" as defined in Section 11165.6 ofthe Penal Code and abuse against any of the other persons describedin paragraph (2) or (3) means "abuse" as defined in Section 6203 ofthis code.   (c) The nature and amount of contact with both parents, except asprovided in Section 3046.   (d) The habitual or continual illegal use of controlled substancesor habitual or continual abuse of alcohol by either parent. Beforeconsidering these allegations, the court may first requireindependent corroboration, including, but not limited to, writtenreports from law enforcement agencies, courts, probation departments,social welfare agencies, medical facilities, rehabilitationfacilities, or other public agencies or nonprofit organizationsproviding drug and alcohol abuse services. As used in thissubdivision, "controlled substances" has the same meaning as definedin the California Uniform Controlled Substances Act, Division 10(commencing with Section 11000) of the Health and Safety Code.   (e) (1) Where allegations about a parent pursuant to subdivision(b) or (d) have been brought to the attention of the court in thecurrent proceeding, and the court makes an order for sole or jointcustody to that parent, the court shall state its reasons in writingor on the record. In these circumstances, the court shall ensure thatany order regarding custody or visitation is specific as to time,day, place, and manner of transfer of the child as set forth insubdivision (b) of Section 6323.   (2) The provisions of this subdivision shall not apply if theparties stipulate in writing or on the record regarding custody orvisitation.