In the background, the outfielders are running up and down the pitch to get the blood flowing.

In the foreground, goalkeeping coach Simone Farelli is making sure that his charges, especially starting goalkeeper Mile Svilar, are well drilled.

I think I'd prefer to be doing the goalkeeping practice, myself. It looks like more fun than just running.

On an unrelated note... why are there so many empty seats with only half an hour to go to the game?

Because they weren't supposed to be empty. Later in the night, they wouldn't be. The attendance was 56,522 for a ground with a nominal capacity of 60,000.

But at the time this shot was taken... well, let's look at a news report that came out that evening:
"Western Australian football fans have been rocked after train chaos in Perth left thousands of fans stranded ahead of a friendly between AC Milan and AS Roma at Optus Stadium.
More than 50,000 fans were expected to attend the match but 45 minutes before kick-off, half of Perth's train lines were hit with unexpected technical issues."

So... a lot of folks were rather late.