The back panel of a real deck lists its outputs. Audionix does the same. In REAR you'll see an Outputs section that shows what the phone is connected to: a USB-C DAC, wired headphones or Bluetooth.
The path
Sound in Audionix travels FILE, then DECODE, then EQ, then OUTPUT. The FILE tab draws that chain and shows the current output path, so you can see whether the EQ is in the way or bypassed.
The EQ and your hardware
The 10-band EQ uses your phone's built-in audio effects. Behavior can differ by phone and by output, which is one more reason to glance at the output path when something sounds off.
Which output suits which tape
We'd be making it up to tell you a USB-C DAC will transform your library. What we can say is that a high-resolution file (the Type II and Type IV tapes in the tape types) has more detail to give a good chain than a compressed one does, and the app makes it easy to see what you're feeding it.
Bluetooth is convenient and wired is simple. Audionix works with whichever you've got.