The Onyx Go is about the size of a 9v battery, but contains a microphone, headphone amplifier, rechargeable Li-ion battery, signal processing (5-band EQ, Reverb and noise reduction) and it sends its mono signal to your phone by Bluetooth as an MP3, Wav, etc.
It's controlled by an app for Apple or Android phones. The Apple app can be downloaded from the Apple store, but the equivalent app does not appear in the Google Play Store. I downloaded it from here, and works fine - https://apkpure.com/mackie-onyxgo/com.mackie.onyxgo
I've just seen that you can also download it direct from the Mackie website in the downloads section for this product.
I've used it as the ambience mic for an IEM, but it can also be used for blogging, recording phone calls etc. Using the app, you can simultaneously record voices whilst playing background music or sound effects from a file on your phone. You could also use it as a Bluetooth adaptor to connect wired phones to a phone with no headphone socket.
The mic on its own may give the quoted 5 hours of battery life, but when used as a headphone monitor with noise isolation earpieces, I only got 3 hours - OK for a gig, but not for a day of busking, so I'm moving it on. The price includes postage to UK mainland destinations.
It is shipped with basic earphones with a selection of tips, and windshield. It is recharged using a USB-C connector (not supplied). Mackie's website information is here -https://mackie.com/en/products/microphones/onyxgo/onyxgo_mic.html?srsltid=AfmBOooRHn-OUI4CS7hGhmtwWT0yCaVDuy_kXvSFQoURHu9Ohf0HiiJ_