Interesting, do you have some reviews on it? There are quite a lot of similar apps on the market. I remember when I was younger and I would make these videos for my friends' birthdays and post them on Facebook, it would take a few tries because they violated copyright claims and in the end it would be posted without any audio. I think that's what made me discover the world of royalty-free music, and I would sure hope that app uses such audios.
There are many tutorial videos on YouTube on how to add background music to a video. In fact, this is very easy to do if you have at least a basic knowledge. In the past, I have often made similar videos for the holidays. First you need to find the free royalty sound effects that you want to install on the video according to the instructions. As a rule, background music creates a special atmosphere that helps to get into the meaning of the video. Unfortunately, some applications provide a basic list of audio tracks for editing, which limits the user's ability.