(I'm assuming we're talking about unprotected left turns.)

I don't know if I ever saw it happen, myself, so I can't say for certain. My understanding of the SDC's logic is that if it was already in the intersection, it would complete the turn, and then pull off to the right shoulder to let the emergency vehicle pass. If it hasn't yet entered the intersection and detects siren lights behind it, I believe it will turn on the hazard lights and remain stationary unless honked at (I could be mistaken, but I think it'll recognize being honked at by emergency vehicles, and will assume it to mean "move forward and clear a path"). The SDCs have an array of microphones around the car to detect honks, sirens, nearby crashes, etc, and can tell the direction the sounds are coming from for this purpose.

That said, because it's listening for sirens, the SDC will usually be aware that there's an emergency vehicle heading toward it well ahead of time, and if they've got their lights on, the SDC will usually be able to determine which vehicle, specifically, is the emergency vehicle, so it can monitor its trajectory and make sure it's staying out of the way when possible. Typically, they will be proactive about steering clear of anything with lights/sirens running.

This would also considered a higher-priority event, and usually it will automatically ping a live human to remotely monitor the situation, and depending on the specific context, they may either command the SDC to remain stationary, proceed forward, make a U-turn, or whatever else may be necessary. In case the emergency vehicle has a loud speaker, we'd be able to hear any requests they're making of us, as well.

For what it's worth, I know that Waymo also works pretty closely with the Phoenix PD, and provide them with updates about any significant changes to the car's behaviors or any tips/tricks for dealing with a stuck car in an emergency situation, so if a situation got particularly sticky, the cops would know how to work around it. My understanding is that Phoenix PD has generally been very cooperative, though they've apparently had issues with state troopers who don't seem to care to learn about how to deal with the cars.