Generally speaking, the 'audio ground' is also the 'earth ground'. If you use an ohmmeter and measure the resistance between the two, you'll often find a (nearly) dead short. They're connected internal to your equipment. Sometimes it is metal-to-metal, sometimes they're connected with a small capacitor. Your ohmmeter would report a capacitively coupled ground as 'open' because to DC, it IS open. But to AC signals, this capacitor is basically a 'short circuit' (i.e. it doesn't break the ground loop you're worried about, it protects you from electric shock if your gear has a massive failure)
This means you might have SEVERAL 'ground loops'. Each piece of gear has an AC safety ground, and each has an audio ground. If they're all tied together, well....
However, my understanding is that ground loops are created by connection of devices in the same signal flow path to separate AC power circuits (as shown in the image below), rather than by the internal low voltage patching of the devices
"Ground Loops" are literally what the name says .. A loop. Anywhere you have a full loop of metal, and any external AC fields (i.e. anywhere in the industrialized world) there WILL be an induced voltage around the loop. The EFFECT this has on your equipment is what's hard to determine. There's a big pile of 'it depends', and no single internet post will teach you how to know when it matters and when it doesn't. I mean, do you live on a farm, or do you live next to a radio transmitter??? Or maybe a hydroelectric power plant???
One key to controlling ground loops when they're unavoidable is to minimize the area of the enclosed loop. Bigger area = more coupled noise. If you were to wire all your AC stuff in a 'star' config, but you don't do same with your audio signals, you can end up with loops regardless.