How can we stream audio from multiple apps simultaneously to a single sound output on iPhone (such as headphones)?

I would like to be able to play multiple audio sources on a single device and single output. For example, music AND an audiobook, or music AND a YouTube video. I previously had this functionality on Samsung using the native “Sound Assistant” app. You had to search for this app, but once you found it, you could play multiple audios at once, individually adjust the volumes of the different audio sources, and adjust the output location. You could have the audio all routes to a single headset, or you could send the audio to different connected outputs. I’ve seen things asking for this but no clear updates. Is this something in the works as an “accessibility” feature? Can an app be made for this or would it need to come in the form of an iOS update? Currently my issue is that the apps all pause audio that was playing if I start new audio. Is this an iOS problem or just a problem with these apps on iOS?