It seems to me that for some departments you have to complete the academy prior to applying for a position, but for others you have to be hired first before you go to the academy.
I live in Virginia and here you have to get hired first before you go to the academy.
However, I keep hearing some people say that they first go to the academy and then apply.
Can someone explain?

Some places allow you to enroll, and pay tuition to attend a police academy. Many smaller police departments in some states require you to have already graduated from a police academy before they will hire you. Most larger police departments have their own academy or share an academy, and will send you to it after you are hired.

Where I live there are very few police departments that will hire you if you haven't already been to the academy. I had to pay my way through. If there are departments hiring in your area that will put you through the academy, I would try that first. It will save you some money if nothing else.

According to their website, if you meet the requirements, you could attend the New River Criminal Justice Training Academy in Virginia before being hired by a law enforcement agency. The New River Criminal Justice Training Academy is one of the Commonwealth of Virginia’s ten regional academies providing a wide variety of criminal justice programs.

Pre-employment training is designed to allow individuals who are not employed by a criminal justice agency to attend state mandated training. Upon successful completion of training, individuals may make application to criminal justice agencies for full time employment. Individuals who are interested in making application to the New River Criminal Justice Training Academy Pre-Employment program are encouraged to discuss this matter with the Chief of Police or Sheriff of the jurisdiction in which they apply. Students are required to pay; tuition, fees, purchase certain equipment, and complete program medical requirements and psychological evaluation prior to matriculation.