Although the model is limited, it is possible to record the ambient sound in stereo with the built-in microphone of the iPhone. As of July 2020, stereo recording with the built-in microphone is only possible when shooting with the standard camera app on the iPhone XS or later (XS/XS Max/XR/11/11 Pro/11 Pro Max/SE 2nd generation) I can do it.
IPhones after the XS have four built-in small microphones, one each in the center of the front notch and near the rear camera, and one each beside the Lightning terminal. Stereo recording is realized by assigning two of these to the left and right channels.
However, stereo recording is possible only when shooting video with the standard camera app. Even with the same standard app, "Voice Memo" does not support stereo recording with the built-in microphone, and will be monaural recording. Since the function of stereo recording using the built-in microphone is not disclosed to the third party, any app available in the App Store will record in monaural with the built-in microphone.
Even when shooting video with the camera app, monaural recording will be performed unless the "Record stereo sound" switch is turned on in the "Settings" → "Camera" screen. The camera app does not have a function to switch the recording mode, so if you want to change to monaural recording, you can only return to the "Settings" → "Camera" screen.
By the way, with iOS 14, which is scheduled to be released in the fall of 2020, the function of stereo recording will be exposed to a third party. When using the rear camera, the rear microphone is assigned to the left channel and the front microphone is assigned to the right channel, and when using the front camera, the rear microphone is assigned to the right channel and the front microphone is assigned to the left channel, and the iPhone is held horizontally and the rear camera is used. When you do, an application such as the microphone on the camera side will be the left channel and the microphone near the Lightning terminal will be the right channel.