Part-holding is a skill that is typically only measured during the callbacks for a group. Callbacks will take place after all groups have held their auditions. Each individual group will reach out to the people whom they wish to call back. Callbacks will have a couple of parts, but part-holding is the main characteristic that may be unfamiliar to some.
During your callback, groups will teach you an excerpt of a song (or multiple) from their repertoire in order to hear how your voice blends with the other group members.
Part-holding is a difficult skill to practice by yourself, so it may be helpful to illicit the assistance of a music-familiar friend! However, there are still many ways to work on your part-holding abilities by yourself.
Try to find songs that you're very familiar with, such as common pop songs with simple melodies. There are many videos on Youtube and TikTok that can help you practice these! Searching something like "harmony practice songs", "harmonize with me", and other similar things will give you many results. In these videos, the person/people will give you your starting note to sing the melody of the song they're using. Then, they'll sing the harmonies, leaving room for you to try to hold your own on the melody line.
While this only allows you to practice holding familiar melodies, it will be a helpful exercise if you aren't used to being the only one singing a part of a song. Try to turn up the audio enough that you are singing at an even volume with the person/people in the video.