First, make sure your Instagram app is up to date in the App Store. The same way you add to your Instagram Story, you can find the option to create a reel at the bottom of your screen. On the left side, you'll find the editing options for your video, including audio, effects, timer, speed, and align to create seamless clips.
The same way you can add a song to your story, you can add a song to your reel from the library of music available in the Instagram app. You can also create the video using your own original audio. Your audio can be used by your followers and credited to you in their own Reels if your account is public.