I've only seen a couple of strongman events in person. They had barbells off to the side for the guys to warm-up with. Mostly there was a light bar and a medium bar that the guys were using, I didn't see any weight changing going on.
But the event was a 550 lb deadlift for max reps. Nick Best was the guest judge at the competiton and he walked up cold to the 550 lb bar and kicked out a quick 15 reps like it was a warm-up. BEAST!
They also had a couple of loaded barbells for the guys to use before the Press competition. I did see some plate changing on those. Then they started the Press competiton with a 220 lb log and moved up from there.
For other events like Farmers and Yolk Walks they'd load it up and the guys would do a bunch of picks with the weight.