How do I use Android Support for Hearing Aid (ASHA) on a capable phone?
Based on this developer oriented video:
Accessible audio: Android hearing aid support and the audio framework (Google I/O'19), the speaker noted that while apps can provide enhancement to the experience, an app isn't required.
That said, if you are trying to create a ASHA compatible device, you should watch the full video, as it does point out the Android ASHA spec:
https://source.android.com/devices/bluetooth/asha
and note that testing seems more involved as the speaker at end describes 'reference device hearing aids' provided by Google and specifically calling out to use the Android Open Source Project (AOSP) codebase to ensure compatibility.
Also be aware that in the Android Compatibility Definition Document (CDD) which defines what is needed to obtain the Google Play Store that hearing aid support is optional.
From: https://source.android.com/compatibility/11/android-11-cdd#7_4_3_bluetooth

If device implementations include support for Bluetooth LE and Hearing Aids Profile, as described in Hearing Aid Audio Support Using Bluetooth LE, they:

[C-5-1] MUST return true for BluetoothAdapter.getProfileProxy(context, listener, BluetoothProfile.HEARING_AID).


So what does this mean as a end-user?
Given the quirks of Bluetooth on Android, wait for hearing aid manufacturers to provide a list and even then supporting manufacturers like ReSound have fine print:


Full streaming compatibility with Android devices requires minimum Android v10 and Bluetooth 5.0 and that functionality has been enabled by the phone manufacturer and service carrier. Accordingly, the compatibility information provided should be used for reference only. Although ReSound strives to achieve seamless device compatibilities, ReSound cannot guarantee full compatibility with all Android devices.