While a desktop computer is recommended, Ecamm Interview guests can join on an iPhone or iPad.
External microphones can be used with Interview mode, provided they connect with an 3.5mm headphone/microphone jack. Since most iPhone and iPads do not have a headphone jack, these microphones usually need to be connected using a Lightning-to-3.5mm adapter (for iPhone) or a USB-C-to-3.5mm adaptor (for iPad Pro).
You will then see an option in the Interview settings screen for Headset Microphone.
Microphones that connect via USB typically need to be used with a proprietary app, and are not detected by iOS and iPadOS web browsers.