Stream audio from a smartphone to another smartphone over a Hot-spot based network The quandary is how! I want to play the audio from one phone to all other phones within the network (or even one of the phones). It is totally fine if I will have to download some application on all of the devices.

What I tried so far is BubbleUPnP. This did the job just fine (from my Huawei P20 Pro to my Y5 II), save that the interface is awful and the limitations arr endless of an amount--with maximum 16 songs in a playlist and maximum uptime of 30 minutes streaming per app launch.
After a couple of hours of additional research, I found the solution myself. From everything that I've tried, SoundSeeder (mobile application) seems to be the best. It's super advanced (including delay compensation).