If you are on iOS and your audio interface has a lightning adaptor, just plug it in, and you’re good to go.
For USB devices: All you need is the appropriate adaptor to adapt your USB interface to your phone/tablet. Also, your audio interface needs to be “class compliant” – which is another way of saying one that you don’t need to install drivers to use. Most usb interfaces are “class compliant”, but I would definitely check before buying.
If you have a lightning connector on your phone/iPad, you will need Apple’s “Lightning to USB 3 Camera adaptor” – it’s expensive, but all the cheap copies on ebay have problems, pretty much without exception – if you buy one and it doesn’t work, I told you so!
If you have an iPad with a USB-C connector, all you need is any old USB to USB-C adaptor – it should cost $3-9 on amazon or ebay.
If your android has a USB-c connector – you just need any old USB to USB-C adaptor as shown above for iPad.
If your phone has the older micro-usb, you need a USB OTG cable, that looks like this:
IMPORTANT: On android you will also need to disable “OpenSL” in koala’s audio settings to make and audio interface work correctly.