Writing not very helpful comment, but perhaps someone will see in future and can help with orientation.
I think it is something connected with the Android version and application restrictions boys.
I have Neutron on few devices and i see the Hi-Res Bluetooth option under Generic Driver on all, except on one with Android 4.4.
On the ones with 9 and 11 appears. Don't have his exact device, but Samsung Tablet yes and on it the Hi-Res Bluetooth option is there.
This phone perhaps is with Android 13 ?
I think is best to write to Neutron Developer, yes. Be sure to write him what Android is and exact model of the phone.
What you can try is open Settings on the phone, search for "developer options" and in the list there find "Bluetooth audio codec",check what available Bluetooth codecs are there. Should see all listed - SBC, AAC, Aptx, Aptx HD, LDAC and whatever else there is. If you don't see the "developer options", go to phone settings, then "about phone" (at the bottom of the settings screen). Enter it and then find and tap 5-6 times quick on "build number". Used to be that way. And then change the codecs while the Fiio is connected. Try some different ones and see if the Fiio shows that different one is playing. For example if you select AptX in the developer options of the phone, AptX should be displayed on the Fiio screen. If LDAC is selected on the phone, LDAC should be displayed on the Fiio and so on.
I see on the previous page his screenshot shows LDAC on the Fiio, which to me means the phone has detected the best possible codec that the Fiio supports and no need to worry. It is playing with the highest possible quality.
For the question if there is sound difference - yes, there is. But only compared to SBC codec. You don't want to use SBC codec (the effect is like listening MP3 160kbps vs more. It does not sound that bad, but it is noticable for sure compare to the others). AptX and the others are good enough. If the Bluetooth range is short with LDAC (or the battery life), you can use AptX. Generally though, the Bluetooth is lower quality than using USB Cable and as Kentajalli said. With this Fiio you want to use LDAC, is what is best with i guess, but AptX and the others will be fine as well. Just avoid SBC.
The image is from the internet, i don't know if it looks exactly like that on Android 13, but these are the options that one want to check.
Yes, I have the same settings and I have done both.
My phone is Galaxy S23 Ultra, Android 13. Maybe I should email the developer about the problem.