Length of stay is calculated based on an animal's status. While animals are in a status that is set to Yes for In Custody, then each day will count towards their In Custody length of stay. If they are in a system default status such as "Healthy in Home" (which is an out of custody status an animal is placed in automatically after they are adopted, returned to owner, or serviced out to a person record) then the count for In Custody does not go up.
Please note that the only time you would want to create a status set to No for In Custody is when you'd like to use that status for pre-intake records.
Similarly the On Site, Holdable, and Adoptable/Available calculations depend on how each status is configured based on those categories. For example, foster statuses are usually set to No for Onsite and Yes for In Custody so that when an animal is in a foster status, their Onsite length of stay does not increase, but their In Custody length of stay does go up. You can then see your length of stay for animals onsite separately from your length of stay for all animals in your custody.
Learn about statuses here and navigate to Configurations > Processing > Status Manager to view/update these configurations.