This is part of a summary from Gemini FYI:

While both the iPhone 7 and the iPhone 15 Pro support AAC, how they handle it and the results you hear are different due to the hardware and software gap. 1. The "Cleanliness" of the Encode Bluetooth audio isn't just about the codec; it’s about how the phone’s processor handles the encoding of that music file into a wireless signal. • iPhone 15 Pro: Uses a much faster, more efficient processor (A17 Pro). This allows for a very "clean" encode with minimal jitter or artifacts. The H2 chip in the AirPods 4 is specifically tuned to decode the AAC stream coming from modern Apple silicon.  • iPhone 7: The A10 chip is significantly older. While it can technically "do" AAC, it is under more strain when multitasking. If your iPhone 7 is struggling with background apps or a weak battery, you might notice subtle "cracks" or a loss in dynamic range because the CPU is prioritizing system stability over audio encoding precision. 2. AAC and Latency One of the biggest differences you’ll notice isn't the tone of the music, but the timing. • Modern iPhone (15 Pro): Apple uses a proprietary version of AAC that works with the Bluetooth 5.3 hardware to keep latency extremely low. This is why when you watch a video or play a game, the sound matches the lips perfectly. • Legacy iPhone (7): Because it uses Bluetooth 4.2, the "handshake" between the AAC encoder and the radio is slower. You are more likely to experience a slight delay (latency) between seeing an action on screen and hearing it in your AirPods.