From what I can find on the Phonak site, your Marvels use Bluetooth 4.2 for streaming. Finding real details is challenging as it is for other hearing aids and bluetooth devices in general. Classic bluetooth usually uses the A2DP profile to stream audio using a codec that both devices (source and sink) must support. SBC is a required codec - there are a number of others (optional, not required by A2DP) that are used for nominally higher quality transmission, usually with an increased bit rate. Bluetooth, including EDR, has a relatively low bitrate for reliable streaming, especially if the devices more than a few meters apart. So all the normally used codecs compress the signal to reduce the bitrate from what it is for uncompressed data.
Uncompressed CD (16-bit, 48 KHz) - 1,536,000 bits/sec
24-bit, 96 KHz stereo - 4,608,000 bits/sec
24-bit, 192 KHz stereo = 9,216,000 bits/sec (the highest bitrate from an uncompressed Primephonic file).
By comparison, the approximate stereo bit rates for the typical codecs, all of which are compressed by a lossy algorithm):
SBC : 328,000 (44.1 KHz sample rate) or 345,000 (48 KHz)
AAC (used by Apple): 256,000 to 320,000 typically
aptX: 352,000 (44.1 KHz) or 384,000 (48 KHz)
aptX HD: 576,000 for 48 KHz at 24-bit
LDAC (Sony): up to 990,000 (96 KHz sample rate for 24-bit)
Recent Android versions probably implement some version of many of the above codecs depending on the phone software and hardware. However, the sink device (hearing aids in your case) must also implement the codec so it would be nice to know what Phonak actually uses in the Marvels. It may be an uncompressed codec but that would be uncommon.
Of course, once the codec in the hearing aids decodes the stereo audio stream, the hearing aids process it via their proprietary algorithms to present a signal to each ear that hopefully mitigates your hearing loss. This may include compression unless a music specific program turns it off.